The Ultimate Guide to Pool Chlorine

Welcome students! Today’s topic is one that is near and dear to my heart – pool chlorine! Chlorine is the most common chemical treatment used to kill bacteria and other disease causing organisms.

Chlorine has made it possible for millions of people to enjoy the benefits of swimming without concerns about pathogens that may be lurking in the water.

Not the most powerful halogen sanitizer, but it is the most common. The abundance of raw materials and ease of manufacture has made chlorine the most popular pool water treatment!

Types of Pool Chlorine

When it comes to Pool Chlorine the most common types are Calcium Hypochlorite (powder), Sodium Hypochlorite (liquid bleach) and Trichlor tablets. These types of chlorine are used for destroying contaminants in the pool water. In practice, chlorine tablets are used for everyday chlorination, because they dissolve slowly, unlike powder or liquid chlorine types, which are used for occasional super-chlorination.

Granular chlorine is used to disinfect and oxidize bacteria by killing contaminants in the pool, a process known as ‘shocking the pool’. This is done by adding enough chlorine to your pool to reach a level of 10 – 30 ppm. To shock your pool, a granular or liquid is used to raise the chlorine level quickly – tablets would never dissolve fast enough.

What’s the Purpose of Pool Chlorine?

chlorine-element

Using chlorine in your pool is one of many ways to keep the water clean, clear and safe for swimming. Not only does it kill harmful microorganisms and bacteria in the water, it also deters algae growth and helps control organic wastes like sweat, urine and body oils that end up in the water.

How Does Chlorine Work?

When chlorine is added to pool water, it breaks down into smaller chemical compounds, namely hypochlorous acid (HOCl) and hypochlorite ions (OCL). Testing for free chlorine (FC) will tell you the parts per million (ppm) concentration of these compounds, which will also indicate the disinfecting power of the water in the pool. FC levels should be somewhere between 1.0 and 3.0 ppm at all times.

Once these these smaller compounds meet up with microorganisms in the water, they destroy the interior and exterior structure of the cell, rendering organisms like bacteria harmless. Both HOCl and OCL aid in disinfecting the pool, but HOCl works much faster than the OCL ions.

On the other hand, when these two compounds come in contact with organic wastes like oils, sweat or urine from swimmers, they react to form combined chlorine (CC), also called chloramines. When the CC levels in a pool get too high, a strong chlorine odor is noticed in the pool area. Chloramines can also cause skin and eye irritation for swimmers. Contrary to popular belief, the strong smell of chlorine near a pool does not indicate a sanitary pool; it actually means there are too many chloramines (and possibly un-sanitary).

water testing

If testing for total chlorine (TC) and free chlorine, you can subtract the FC reading from the TC results to find the amount of chloramines in the pool (TC – FC = CC). Ideally, the TC and FC numbers should be the same, indicating a chloramine-free pool. To boost FC levels in your pool and break down chloramines, just add more chlorine! Superchlorinate the water to give the pool a proper shock treatment.

Chlorinated Isocyanurates:

trichlor chemical - image by commons.wikimedia.org

These are a group of chlorine products that contain cyanuric acid, which helps protect chlorine from degrading in sunlight. Two of the most common isocyanurates used in pools are Trichlor and Dichlor.

TriChloro-S-Triazinetrione – Cl3C3N3O3

trichlor-molecule

The King of chlorine products, “Trichlor” is pressed into 1″ and 3″ tablets, and also in stick form. Trichlor contains 52% of cyanuric acid by weight, and contributes 0.6 ppm of cyanuric acid is added to the water, for each 1.0 ppm of Free Chlorine added.

Trichlor has a relatively low pH of just under 3 – which will require the addition of pH increaser to maintain pool pH in the 7.4-7.6 range.

Trichlor has 90% available chlorine, which means that chlorine tablets are 90% as strong as chlorine gas, which has 100% available chlorine content.

Trichlor is capable of carrying other ingredients to control of algae or to improve water clarity, however this will reduce the percentage of available chlorine. For example, our Triple Action Tabs have 72% available chlorine.

Application: Trichlor tablets or sticks dissolve slowly when placed in water. 1″ tablets dissolve more rapidly due to increase surface area, which is desirable in pools or spas with higher chlorine demand.

Pool chlorine tablets

Trichlor should be added to the pool using a chlorinator. The best type is installed into the pool equipment plumbing, or a floating chlorinator can be used. One should never place chlorine tablets in the skimmer, or toss them directly into the pool.

Precautions: Chlorine tablets are highly reactive, and can explode or ignite if exposed to contaminants or other chemicals, and will release chlorine gas when moist. For this reason, chlorine tablets should be tightly sealed, and stored in a cool, dry and well ventilated area.

Chlorine gas from pool tablets will oxidize nearby metals, and should be stored where this is not a concern. Never consolidate two buckets of chlorine products, slight compositional differences can result in a reaction, producing explosion, fire or toxic fumes.

Sodium Dichloro-S-Triazinetrione – NaCl2C3N3O3

dichlor-molecules

Another type of stabilized chlorine, Dichlor is a granular form of chlorine. Using Dichlor will contribute to the level of cyanuric acid in the pool. By weight, Dichlor is 57% cyanuric acid; for each ppm of chlorine added, 0.9 ppm of cyanuric acid is added to the water.

The pH level of Dichlor is nearly neutral (7.0), and won’t affect the pH of the pool water noticeably, is fast dissolving and because it’s sodium based, it won’t contribute to calcium levels in pools.

Dichlor has an available chlorine level of 56% or 62%, depending on the formulation – anhydrous or dihydrate. Supplemental additives can be mixed with Dichlor, such as our Assure MultiShock with clarifiers and buffers, however this lowers the available chlorine percentage to 58%.

Application: Granular chlorine products are used by broadcasting the granules over the pool surface, or by diluting into a bucket of water, and pouring the mixture around the [inside] edge of the pool. Consult the label for dosage instructions prior to application.

Dichlor is used for shocking or superchlorinating, and can also be used for daily chlorination for small pools, spas and fountains.

Precautions: When broadcasting granular chlorine, one should mind the direction of wind that may blow the material in unintended ways.

Dichlor is incompatible with any other type of chlorine and is highly reactive to small amounts of dirt, dust or leaves. Mixing with any other pool chemical can cause a violent reaction, including explosion and fire.

Do not store open packages of Dichlor, which can spill or take on moisture or other contaminants. When using 1 lb. bags of Dichlor, use the entire bag at one time.

Hypochlorites:

hypochlorites image by commons.wikimedia.org

Another group of chlorine products is known as hypochlorites, which are ions composed of oxygen and chlorine. They are made by hydrolyzing salts of sodium or calcium with chlorine, followed by rapid drying and cooling.

Sodium Hypochlorite – NaOCl

sodium-hypochlorite-molecule-

Liquid chlorine, or bleach is the most well known hypochlorite, used for a variety of cleaning and disinfecting purposes. It is not stabilized, so cyanuric acid is added to the pool separately to extend the product.

The pH level of sodium hypochlorite is very high at 13, the highest of any chlorine. This requires frequent addition of pH decreaser into pools that use liquid chlorine as the primary sanitizer.

The percentage of available chlorine is quite low for pool bleach, 12.5% is standard. Household bleach, by contrast is typically a more diluted 5 or 6% solution of the same sodium hypochlorite solution. However, due to low cost, sodium hypochlorite can be one of the cheapest chlorination methods, especially for large pools. Shelf lift is short however, liquid chlorine degrades within 6 months, losing potency over time.

Using sodium hypo can elevate TDS (total dissolved solids) levels over time, due to the high level of sodium, more than solid or granular forms of chlorine.

Application: Introducing liquid chlorine into pool water is accomplished by using a chemical metering pump; one specifically designed for the purpose, which draws chlorine from a storage container, and injects it into the pool return line.

Liquid chlorine can also be hand-fed into the pool, by pouring a bucket full directly into the pool. It dissolves instantly and leaves behind no residue.

Precautions: Storage of large quantities of sodium hypochlorite is likely safer than an equal amount of tablets, and certainly much safer than using chlorine gas. Nonetheless, caution should be taken to avoid contamination of the liquid, spills or leaks in the pumping hoses.

Like other chlorinated products, strong reactions can occur when contaminated with dirt or dust, or when mixed with other pool chemicals. Accidental mixing with acids can create an extremely toxic form of chlorine gas.

Calcium Hypochlorite – Ca(OCI)2

calcium-hypochlorite-molecule

Cal Hypo, as it is colloquially known, is the most common form of granular pool shock and also the least expensive. Not stabilized, and not meant for daily chlorination. Add in the evening when the sun goes down.

The pH level of calcium hypochlorite is high at 11.7, but when used for a chlorine booster or for shock treatment, one should not see any noticeable effect on pool pH levels.

The percentage of available chlorine for Cal Hypo is standard at 65%, however some formulations such as Super Shock deliver a higher potency at 73% available.

For pools with hard water or high calcium hardness levels, keep in mind that Cal Hypo contributes to your calcium levels. For every 1 ppm of free chlorine added, Cal Hypo also adds 0.7 ppm of calcium to the pool.

Application: Like all granular pool chlorine products, it can be broadcasted or sprinkled across the surface or it can be diluted by pouring into a bucket filled with water, and added to the pool.It can also be poured directly over stained areas (plaster pools only), for quick removal of organic stains.

Consult label instructions for dosage, but the standard dose is 1 lb per 10000 gallons, which should yield 7-9 ppm of free chlorine, depending on strength used.

Precautions: Cal Hypo is one of the most dangerous pool chlorine products, to use and store. It is easily contaminated and reacts strongly to any foreign substance, yielding fumes and fires. Mixing with trichlor tablets and water creates a very strong explosion.

Salt Chlorinators

One of the most popular alternatives to chlorine tablets for sanitizing your swimming pool is a chlorine generator. With a salt water chlorine generator, you add pool salt to the water and the salt cell creates chlorine from the (slightly) salty water, through a process of molecular disassociation or hydrolysis. The amount of salt you add to your pool is roughly 20 lbs of salt per 1000 gallons of water.

The advantage of using a salt system compared to chlorine tablets is that there is less odor, and the salt will make the water much softer and provide a buffering to the pH level. But make no mistake, a saltwater pool is a chlorine pool, which is why I have added it here.

Economical Chlorine Use

Chlorine costs can be one of the larger expenses for the pool owner. With the average inground pool using 4-6 tablets per week, pool owners may be motivated to find ways to reduce their chlorine consumption. Here’s some tips to lower your chlorine cost.

  1. Stabilize It! Using Cyanuric Acid in your inground pool protects your chlorine from the sun. Cyanuric acid is sold by the name “Conditioner” or “Stabilizer”. For a residential pool, you want to keep the stabilizer in the range of 30-50 ppm. If you do not have the proper amount of stabilizer in the pool water, the chlorine can quickly degrade from UV rays, and you may have trouble keeping a chlorine residual on sunny days. Using stabilized chlorine tablets, such as In The Swim 3″ tablets is also recommended, to help maintain your stabilizer levels.
  2. Reduce It! If you want to reduce the amount of chlorine, you will need some kind of alternative sanitizer to work with the chlorine. Some of the most popular alternative sanitizers are the Frog System, Nature 2, Ionizers and Ozonators. None of these systems will eliminate chlorine completely, but will significantly reduce the amount of chlorine you need to have in the swimming pool. Adding a supplemental sanitizer will allow you to reduce the chlorine level to 0.5 -0.8 ppm, as they kill bacteria and other organisms in the pool water, reducing the chlorine demand.
  3. Feed It! The best way to use chlorine tablets is to use either an inline or offline chemical feeder. Place the chlorine tablets into the feeder and adjust the dial, to control the chlorine level precisely and prevent over-chlorination. By adjusting the number of tablets in the chlorine feeder, and adjusting the water flow through – you can accurately control the dissolution rate. As water becomes warmer however, and pool use increases, the chlorine demand will rise accordingly.

Shocking Your Pool Effectively

There are many different types of pool shocks available for shocking the pool – Liquid Chlorine, Calcium Hypochlorite, stabilized Sodium Di-Chlor and also a non-chlorine oxidizer called Potassium Monopersulfate.

Pre-Dissolving: With liquid shock or non-chlorine shock, you do not need to pre-dissolve, but when adding Cal Hypo and DiChlor pool shock, to a vinyl-lined pool, it is very important that you follow package instructions, and pre-dissolve the granules into a bucket of water before adding to the pool.

pool-shocks

Shock your pool after a heavy bather load, to kill anything that has been able to resist your daily chlorination routine. Shocking your pool also removes chloramines, or chlorine molecules that have combined with nitrogen or ammonia. Chloramines are no longer effective sanitizers, and are responsible for red eyes, irritated skin and a strong chlorine smell. Shocking the pool is also the best treatment for the removal of algae.

In order to properly shock your pool to completely oxidize the water, it’s important to reach what’s called “Breakpoint Chlorination“. This is the level at which everything in the pool is oxidized, which is generally 30 ppm. If you come close, but do not reach this threshold, the condition may worsen. To reach 30 ppm, this generally requires 3 lbs of shock per 10,000 gallons of pool water, otherwise known as a ‘triple-shock‘ dosage.

Another important tip for shocking effectively is to adjust the pH to the low side of the scale, around 7.2. High pH levels render chlorine very sluggish and suppresses the activity and thus kill rates. When pH levels are over 8.0, as much as 50% of your shock can be wasted!

Pool Chlorine Safety

Here are a few pool chlorine safety tips to help you avoid a Haz-Mat situation.

  • Always add chemicals directly to the pool, or to a bucket of water.
  • Never mix different types of chlorine, or allow contamination from leaves or dirt.
  • No smoking around pool chemicals, some types can ignite easily.
  • Store your chlorine in a cool, dry location – out of the reach of children.
  • When opening chlorine buckets or chlorinators, avoid breathing chlorine gas.
  • Use complete bags of pool shock. Do not store opened bags of pool shock.

61 thoughts on “The Ultimate Guide to Pool Chlorine

  1. Sorry to ask another silly question. Can I go ahead an use my calciium hypochlorite again after I run out of the dichlor?

    • Julia, yes they are safe to use interchangeably, or even at the same time, they should just never come in contact with each other, dry or wet, although you could add one, and then the other to the pool.

  2. Hello this may seem like a silly question but I am used to using certain products. I had a health issue going on where i couldn’t be in a pool and didn’t have time to maintain it. I always use 3 in triclor tabs and just maintain it with sodium hyperchlorite shock every week or so. Up u til now everything has been perfect. But during g my time off we ha e had some very bad storms and not the best of help when maintaining the pool. I had some visible algae yesterday and got in. I noticed the other half bought some pool shock. But he purchased dichlro-s tiazinetrione. Now my pool hasn’t been shocked in a while now no one wants to do it lol. Is it safe to use the dichlor all my level in the pool are super low like zilch practically. Im not used to using dichlor so i don’t want to over do anything nor mix the wrong things. Is it safe to use fichlor with the trichlor tablets? And after wards can I get back to my normal pool chemistry?

    • Hi Julia, think of DiChlor as stabilized Cal Hypo, it has cyanuric acid added to protect it from the sun. Its about 10% less potent than Cal Hypo, but lasts longer during the day. No worries about a reaction with Trichlor, use Dichlor as you would Cal Hypo, you should not notice any difference.

  3. Nikki Shelton

    Hi! I have a permasalt pool and I use calcium hypochlorite weekly to shock it. I continue to have issues with algae, so I bought some trichloro tablets to put in a floater to help out. I read they are very explosive used together. I was told to take the tablet out 12 his prior to shocking and then wait 24 hours before putting the tablet back in. Does this sound accurate?

    • Hi, they are only explosive if they are touching each other dry or put together in the floater (tabs and granular). You don’t have to remove the floater when shocking the pool, or if you do, there’s no need to wait any length of time.

      • Nikki Shelton

        One more thing!! If I add baking soda for alkalinity is there a need to take out floater or not shock close to the time I put in baking soda?

        Thanks!

  4. Carlos Mendez

    I’ve being adding Trichlore for 6 months to a 30,000 gallons pool, with 10% weekly partial water changes, Cianuric acid levels had being increased slowly to 40 ppm at this moment, so I decided to start adding sodium Hypochlorite using a metering pump. Now, since Hypochlorite increases PH, some muriatic acid need to be added on regular bases to achieve 7.2-7.6 PH values, my question is: Can I add this muriatic acid using a metering pump too? can I add by using pumps at same time? Thanks in advance for your attention. Best regards

    • Hi Carlos – you can also add muriatic acid using a metering pump, and both could be running at the same time. The acid pump would use a different feed tube or ‘squeeze tube’, one that is resistant to acid, I believe Tygon? It’s a common option, the tube to use with acid, but would have to be ordered separately as most chemical pumps will come with the chlorine tube. Important to note that liquid chlorine and muriatic acid must never be mixed, or it creates an extremely dangerous type of mustard gas. As mentioned, they can be pumped using separate pumps, and the injection point (in the return line) should be spaced apart several feet, and one must take care to never have the chemical pumps running unless the filter pump is running, to dilute the chemicals with fresh water and prevent them mixing inside of the pipe. also be careful in handling both mixtures, with careful storage practices to prevent spills or any possible contact with each other. But again, it’s a common method, to inject both chemicals, especially on larger pools.

  5. What do you recommend for chlorination and hard water?

    • Hi John – for most people, using 3″ chlorine tablets is the most convenient and best way to add consistent and constant chlorination. As for hard water, there are no products that can remove excess calcium from the water, so you just deal with it. Avoid using calcium hypochlorite pool shock, but opt for DiChlor or Non-Chlorine shock instead. Also be careful to keep your pH level proper, if it gets too high, it’s easier for calcium to come out of solution to cloud the water or deposit on surfaces. Otherwise, business as usual. 🙂

  6. Hi Davy,

    I have been using trichloro-s-triazinetrione tablets in a floater. Noticing the cyanuric acid levels increasing, I removed them and added calcium hypochlorite tablets to the skimmer basket. Almost a week later the chlorine level decreased from 3-4ppm to 1.2 and the calcium hypochlorite tablet was soft but not dissolved at all.
    Is this normal or should I have waited for the chlorine from the trichlor tablets to have completely dissipated before adding the calcium hypochlorite tablets? Is there something altogether different I can do.

    • Hi Lillian, thanks for the question! Trichlor and Cal Hypo tablets should not be mixed, residue from each other can cause a (dangerous) reaction. Cal Hypo tablets must be used in a Cal Hypo feeder, which wets only one tablet at a time. They don’t work in trichlor feeders, because they (usually) turn to mush, and gunk things up. Some other solutions to the high CYA are 1) drain/refill some water %, 2) Use BioActive CYA Reducer, 3) Add a Nature2 Express or Frog purifier, to reduce your chlorine demand, 4) Lower your (normal?) 3-4 ppm chlorine level to 1-2 ppm or 5) Install a Cal Hypo tablet feeder ($$$) and use Cal Hypo ($$) Tablets.

    • Bob Buel

      One thing that you might check is the levels of total chlorine–versus just free chlorine. With extremely high levels of cyanuric acid, It would normal to see the chlorine sanitizer become bound, where it could not sanitize, would show very little free chlorine (because they are now chloramines), yet the total chlorine level would be quite high. The answer is to change out water and use the cal hypo or something that doesn’t have stabilizer, (not trichloro-s), or supplement with minerals/ozone/UV, so as to reduce chlorine demand by half.

  7. Travis Dillon

    I use granulated sodium dichloro-s- triazinetrione blend (chlorbrite) for my 500 gallon hottub. I am tired of adding this everyday or every other day and I was hoping that I could use 1” tablets or sticks to make my life easier. I am allergic to bromine (red itchy bumps) and would prefer to use chlorine for this reason. Is this possible and if so, what do you recommend and what setting on my floater should I use? Thank you!

    • Hi Travis, sure you can use 1″ chlorine tablets. Buy a small bucket (10 lbs is our smallest size), and then add them to a spa tablet floater, probably will use 4-6 tabs at a time, with the baffles halfway open…? You can experiment with different numbers of tabs (variable 1) and the baffle setting on the floater (variable 2), to produce the most consistent and constant level of chlorine, in the range of 2-3 ppm, as verified by your test kit or strips. Once you nail it down, you can relax, refilling the floater every 5-7 days. If you go out of town, you can double the amount of tablets, and close the baffles more, to last up to 2 weeks.

  8. Hey I heard sodium dichloride s triazedine is extremely toxic if I accidently inhale some that what will happen will it harm me in small amounts ? Also if mixed with water does it create gas ? That is harmful toxic and deadly?

    • Hi Craig, what you say is true for any chlorine product, and granular is specifically ‘reactive’, and designed to react with water, or moisture. Inhaling the fumes, or gas-off could cause imflammation of the airways or breathing trouble, usually temporary, but seek medical attention if you ever get chlorine gassed, which is not so uncommon. 1000’s of emergency room visits per year.

  9. Hello, I just purchased some tri-chlor tablets. I’ve always used just he regular chlorine tabs, but thought in the heat of the summer it would last longer. Is there a problem with just starting to use the tri-chlor tabs when the regular ones run out? Do I need to know or do anything differently?

    • Hi Mary, do you mean that you used to use 1″ tabs and now you are using 3″ tabs? If you were using chlorine tablets before, chances are you WERE using Trichlor tablets, maybe you just didn’t know. THere are also Cal Hypo tablets, but they require a special feeder ($$$), and one that Trichlor tabs cannot use. If you used Cal Hypo tablets in a regular pool chlorinator (Hayward, Rainbow, King), then you should not use the chlorinator for Trichlor, but should replace the feeder. But, I think you may have been using Trichloro-S-Triazinetrione before, perhaps?

      • Hi, Davy. This is the first time I’ve purchased anything called Tri-Chlor. In the past I have just used the 3″ chlorine tablets and I put them in the skimmer. I bought a floating chlorinator and planned on putting the new tri-chlor tabs in it. Is there any reason why this wouldn’t be appropriate having used regular chlorine tablets in the past. I still have several to use up before trying the tri-chlor tabs. Thanks.

        • Hi Mary, that sounds solid, use the Trichlor tablets in the floater, I see no problem with any reaction from other chlorine types used previously.

  10. Gale Coleman

    Hi Davy,
    I’m back with another question- about what sanitizer to use with a hot tub. We aren’t in it that much, but had been using the trichlor tablets and the the stabilizer is getting higher and I don’t want it to keep going up. I purchased the chlorine free oxidizing shock (thanks for the speedy delivery)but am wondering if I should be doing something in addition to that. What can I use that doesn’t have the stabilizer in it? Or is the shock enough since we aren’t in the hot tub that often?
    Thanks again for your help.

    • Hi Gale, you can use Bromine Booster, to add sodium bromide ions to the water (start with fresh water), then use shock to activate or convert the bromide into bromine. You can use chlorine or non-chlorine shock, and you should be able to test a bromine reading, target = 3-5 ppm. You could also use bromine tablets, in a small spa floater. Bromine is best for spas, Trichlor is best for pools…

  11. Eddie G.

    Hi, I have a question. I have a vinyl liner, 1,500 gallon swimming pool. I stared using sodium dichloro-s-triazinetrione granuals to sanitize the pool. I’m finding this to be inconvenient and was wanting to switch to a floating chlorinater using trichloro-s-triazinetrione 1″ tablets. I live in Phoenix, AZ and was wondering if this would be safe. I read above the sodium dichlor is incomparable with any othe chlorine so I was wondering how to go about this or if I need to start completely over and drain the pool. I don’t want to create any dangerous chemical reactions. Thanks!

    • Hi Eddie, no worries, they are only a problem if they are mixed when dry, they should never touch each other or fire or explosion could result, along with noxious fumes. But you can use tablets in the pool for everyday chlorination, and then use the dichlor for shock treatments or for a quick chlorine boost when needed, no problem.

  12. I have an above ground vinyl pool (5000 gal) that I just filled 4 days ago. I put trichlor tablets in my floater, and did a cal hypo shock.
    No chlorine on test strips. Alkalinity was low so I added ph up (calcium bicarbonate) and another full dose of cal hypo shock (these were purchased last August and stored in the basement).
    Still no chlorine reading. Ph is good, but alkalinity still low. Decided I needed to learn about what I was doing 🙂 so started reading online. Added 2 lbs baking soda. Alkalinity now ok. Then added chlorine bleach last night which brought chlorine to 3ppm. Cyanuric acid a little low, but close to 30. Ph 7.8.
    So now I have used cal hypo, trichlor, and chlorine bleach. Is this okay????? Where do I go from here? I would like to shock the pool due to first opening, and rain and tree debris.
    After shocking can I go back to trichlor tablets or should I stick with the chlorine bleach?
    Thanks for your website and any advice.

    • Hi Gale, tablets, shock and bleach all produce the same active ingredient, hypochlorous acid, the killing form of chlorine. It’s OK to use all 3 types (but they can never be mixed together in a bucket or bag, or… it will literally explode and/or ignite. Sounds like you are OK at this point, tablets should be able to maintain the chlorine level. Tablets are preferred to bleach, unless you have a bleach pump and injectors, to slowly feed it thru-out the day, so that a constant and consistent chlorine level can be maintained.

      • Gale Coleman

        Thanks for the input. The trichlor tablets I’ve been using hardly seem to be dissolving. I think they are 1-2 years old. Would age affect their efficiency? By this afternoon all the chlorine was gone (sunny, hot day, with 12 ml clear solar cover) I had @ 10-12 1″ trichlor tablets in the floater.
        I’m thinking I will replace the trichlor tablets. Any thoughts on this?

        • Hi, tablets should hold their potency for several years, although heat, moisture and a loose bucket lid can cause some loss of strength. Although I don’t normally recommend this, try dumping the tablets in the skimmer basket, so they will dissolve faster (remove them if the pump is shut off for longer than a few minutes, so they won’t continue to leach). You said you cyanuric acid level was at 30 ppm? That should be good to hold the chlorine in bright sun… I guess you could buy some new tablets – if you will be needing some anyway later in the summer, to test the theory. The pH is a little high at 7.8, chlorine is much more active at 7.4. And, I’m wondering if the test kits are new? If from last year, perhaps they cannot be trusted… ? Not being able to get a chlorine reading can occur when the chlorine demand is higher than the chlorine, often some invisible nitrate or phosphate – not saying this is your case, but many pools need a major shocking to break thru, and finally establish a chlorine reading, in your case that may be like 3 lbs of shock (all at once)

          • Gale Coleman16

            Thanks Davy,
            All is well now. Like you said, it appears that is just needed a Big shock. Trichlor tablets are now doing just fine.

            One more question- but for a hot tub- what would you use as the chlorine source? We changed the water in April, and use trichlor tablets. Hadn’t paid much attention before to the stabilizer until now, and see that it is on the high end of ok. I was looking for lithium hypochlorite to purchase, but don’t see it. Any other suggestions?

            Thanks for all your help- you are now my pool supply store!

          • Hi Gale, for the hot tub, or a pool that is covered most of the time, you don’t need cyanuric acid, because there is very little sun. CYA can get in the way of sanitation because it reduces the activity of chlorine, so when CYA gets high (50-100 ppm), it can suppress chlorine’s killing power enough to the point where the water could be unsafe, unless you are also raising the chlorine level much higher. For example, if there is 90 ppm of cya in the water, a safe chlorine level may be 7-8 ppm. But for spas that are drained regularly, the build-up is not usually an issue, with replacement water starting again at zero. Lithium hypo is no longer available, it got too expensive to make – non-chlorine shock is the next best substitute, which has most of the benefits of Lithium shock, except the chlorine…

  13. I have been using Trichlor tabs through a feeder for primary chlorine dosage and powder shock treatment after heavy use and have done pretty good with FC levels. However, the CYA levels have been over 100 for the second year now. My CH tends to be near the high side of normal and my pH tends to be on the low side of normal. I am trying the Bio-Active CYA Reducer but have not seen any change (1 pack last year, two this year), so other than draining the pool, I would like to try a non-stabilized chlorine product and try to dilute the CYA over time. I have looked at sodium hypochlorite but am looking for guidance. What is my best option?

    • Hi Chris, the best option is draining and refilling half of the pool, unless you are under a water restriction, or it is very expensive – in most parts of the US water is under $5 per 1000 gallons. If you are on a well, you can use that water too, it won’t harm the well to run a hose non-stop, unless your well output is known to be very low or limited. But if not possible to drain and refill half of the pool, then you can use a nature2 express purifier, to reduce your use of tablets, cutting it in half, which will reduce cyanuric acid input, to half as well. You can also use liquid chlorine with a chemical pump. Cal Hypo tablets are another possibility, but they are pricey and require a cal hypo feeder that only wets one tablet at a time.

  14. can you add sodium hypochlorite to a pool that has
    trichloro-s-triazinetrione without problems?

    • Davy Merino

      Hi Steve, sure as long as they are separately added of course, don’t let the two come in contact or things could get scary! But yes, no problem to use both chlorine types in the same pool.

  15. Todd Menna

    I know that mixing trichlor tablets with an acid, such as muriatic acid, can generate chlorine gas. Do you know if that gas generation will also build pressure, for example if it occurs within a chlorine feeder and the cap is placed on it? Could it lead to an explosion, or will the pressure regulate and the reaction stop?

    • Davy Merino

      Hi Todd, mixing trichlor and acid will actually create a kind of mustard gas that will quickly kill if the fumes engulf you. Gas buildup from a reaction inside of a chlorine feeder could blow the lid off and/or split the cylinder, violently, injuring or killing someone standing over it. Now mixing two types of chlorine and adding water can produce an instant explosion from the reaction, which is not a gas pressure problem but more… explosion. So never mix different types of chlorine.

  16. Hi Davy,

    My pool is cloudy and I have been using the tri- chlor tabs. My pH was low and my chlorine is too. I have added to raise the pH. Can I add liquid household bleach to bring up the free chlorine?

    • Davy Merino

      Hi Paula, yes you can use unscented regular bleach in the pool. You would need about 3 cups, per 10000 gallons, for each 1.0 ppm that you want to add to the pool. Adding 1 gallon of 5% bleach, per 10000 gals will take free chlorine up to around 5 ppm. Cloudy water can also be symptomatic of filter problems, or phosphate contamination, or old water with high solids content.

  17. Valerie Brown

    I am very confused about my pool water. I can not get the chlorine level up, I have added about 24 gallons of liquid chlorine. I have added many packets of shock (about 20). My ph is low and alkalinity is high. That confuses me. My cyanuric level is also low. My pool water is no longer cloudy green but clear but the chlorine level is still showing none.

    • Hi Valerie, if your test kit or strips are not expired and results can be trusted, the ‘chlorine demand’ must be higher than the amount being added – something is consuming the chlorine! :-/ Low pH and low cyanuric (20-40 ppm) levels, that is good for chlorine levels. Could be a phosphate/nitrate contamination, you can test for phosphates or treat with a remover if levels are testable. Is the pool clean, and the filter running long enough each day (this [hot] time of year that’s 12-18 hrs/day for most systems). You have added a lot of chlorine, not sure what size your pool is, are you also using tablets in addition to the shock treatments? A simple solution that can sometimes help is to replace a portion (20-50%) of the pool water and refill with fresh, where possible, if my other thoughts aren’t helpful.

  18. We have an in ground pool at our property that is about 18,000 gallons. Right now we are using Dichlor but it always seems that our cyanuric acid is too high (around 80-100ppm). What could we be doing wrong? What else could we use to help keep the cyanuric acid down?

    • Hi Meghan, Dichlor does contain Cyanuric acid and for each 1 ppm added to the pool from dichor, 0.9 ppm of cyanuric acid is added. Switch to Cal Hypo shock, which is not stabilized, and the problem should stop. YOu may want to drain and refill half the pool too, to drop into the 40-50 ppm range. There is a new product called BioActive that can be used to remove cyanuric acid, but results have been mixed for users, not always consistent…

  19. mark Heucke

    Hi
    I have a pool that is 15-20,000 gal. Years ago I got it fiberglass lined. I have been using liquid clorine and tri-clor tabs to clorinate. However my local pool store says I should use a shock and triclor. Does this work, or what is best for my pool or should I switch from the liquid to a diclor?
    Thanks!

    • Davy Merino

      Hi Mark, you’re fine with liquid and tablets. Liquid does have a high pH level, and adds salt to the pool, but Dichlor adds cyanuric acid to the pool, and is much more expensive. IF you were going to use shock, I would use Cal Hypo, not Dichlor, just for the big cost savings. I like how liquid chlorine reacts (kills), and for really green pools, I like the synergistic effect (seemingly) from adding both (separately) to the pool, liquid and granular Cal Hypo shock. What is best for your pool, is kind of a non question, they are the same really. They all produce hypochlorous acid, and although all have some side effects, and different costs, they all do basically the same thing. I can’t think of a reason why Dichlor would be better than liquid, I think I would stay just where you are…

  20. Hi there, I am so confused between Di Chlor and Tri Chlor. We have a small above ground pool. Less than 5000 gallons. I read a blog that states “NO CAL HYPO” on vinyl pools. The more I read online, the more I get confused.
    Which tablet is best to use.

    • Hi – tablets are Trichlor, always and only. Well, actually you can buy cal hypo tablets, but most people use Trichlor tablets and Cal Hypo shock. The problem with cal hypo shock is that it is a granular powder that does not dissolve instantly, so it can bleach vinyl liners if poured into the pool without pre-dissolving it first, in a 5 gal bucket of water. A few stray granules are not going to instantly bleach a liner, it takes years of steady abuse actually… If you predissolve, you can use the cheap and effective cal hypo no problem. Now Dichlor, that a stabilized granular form of pool shock, also not recommended for vinyl pools, for the same reason, but no problem if you pre-dissolve. Dichlor has some cyanuric acid to protect it from the sun, but otherwise is similar to cal hypo shock. Lithium shock and non-chlorine shock are both quick dissolving and do not require predissolving, but they are much more costly. Most people use trichlor tabs and cal hypo shock…

  21. I opened my pool and had bad alge growth. I used HTH Super Green to Blue Shock System and my water is blue. I balanced my alkalinity and ph but don’t have a chlorine read. I placed chlorine tabs in my floater but still no chlorine read. Do I add more shock to get a chlorine level or granulated chlorine?

    • Hi Kara, bad test strips or reagents can produce a false negative reading, but what is likely happening is that the chlorine is getting used up, still battling the (now unseen) algae. I would shock the pool again, in the evening, with 1 lb per 10,000 gals, or a normal dose. Check the chlorine level after shocking, and again in the morning. IF still no chlorine, check that your cyanuric acid level is 20-50ppm, If very low or much higher, around 100 ppm, that could be the problem.

  22. Hello, I have been using Dichlor bags to shock my inground 10k gallon pool. Can I use regular chlorine tabs in my feeder with this? or do I have to buy dichlor tablets? I bought Walmarts version of what they pass as pool shock. Was in a bind when opening pool. Had massive issues getting the water blue. When it kept reading at 0 for chlorine (used liquid shock to open) switched to dichlor , and now water is turning blue again. Can i put the chlorine tabs in the tab feeder and use dichlor to shock once a week?

    • Hi, yes put the chlorine tablets (Trichlor) into the feeder, and apply dichlor chlorine by hand, broadcasting over the pool. You can use them together… but don’t let them touch each other, dont put the granular in the chlorinator, or… boom! could be a big problem…

  23. Hello. Is DI-ZAP safe to use regularly in a vinyl in-ground pool?
    We used it last year and i found that it kept our pool sparkling
    clean without having to use a lot of other chemicals, but now I’m confused reading about di-chlor, tri-chlor and lithium chlor comments
    on the web. Can you clarify (no pun intended)? Thank you.

    • Hi Diane, as long as you predissolve the Di-Zap in a bucket of water, stirring to dissolve the granules, it is safe to use on vinyl liners. Lithium is a much more powdery form of shock that releases from the binders very fast, so there is no need to predissolve, the same with non-chlorine shock. But Cal Hypo or Di-chlor shocks should be predissolved. trichlor is the tablet form of chlorine, so that’s different… For vinyl pools, you want to avoid undissolved granules laying on the surface of the vinyl, which can weaken or fade the colors. Once or twice won’t hurt, but if you repeatedly broadcast the shock over the pool, it can affect the vinyl over time.

  24. Bentkowski Donna

    I understand it is very dangerous to mix dry forms of both Calcium Hypochlorite and Trichlor together in a small amounts of water. But Is it ok to use the slow dissolving Tricolor tablets inside ones automatic feeder and occasionally add Calcium Hypochlorite powder to super chlorinate or shock ones pool as needed ?(not adding it to another feeder but just broadcasting the granules over the pool’s surface.)The Tricolor is already dissolved in a large amount of pool water when the dry Calcium Hypochlorite is added. I have read the instructions on the 3 in.Jumbo slow dissolving Trichlor chlorinating tabs container from Leslie Pool Supply and they recommend one add your tabs as stated above and super chlorinate with powder power plus (active ingredient Calcium Hypochlorite)as explained above.I have also asked their store personnel and they say one can do this safely ,do you agree?
    I have been doing this for a few wks and have not seen any explosions.

    • Hi Donna, yes no problem at all to use Cal Hypo in a pool treated with Trichlor tablets. You are right that the two chemicals are highly incompatible, and can explode if they contact each other with water added, in a small space like a chlorinator. But adding Cal Hypo to the pool water will not cause a problem. Just don’t add shock the chlorinator, or allow it to contact tablets (or any other chemical). Keep your chemicals separated, dry, cool and always tightly covered. Thanks, good question!

  25. Bob Rhodes

    I just had a new liner installed and on the manufacturer’s website they say if I am using Trichlor, that I should never use it in a floating dispenser and that I should run my pool circulation 24 hrs a day. https://www.lathampool.com/pool-care-and-maintenance/

    • Davy Merino

      Hi Bob, well that’s a lot of CYA from Latham – millions of vinyl pool owners use Trichlor without problems, or without running the pump 24/hrs per day. However, problems could occur to vinyl if people put tablets in the skimmer basket and don’t run the pump 24/7. When the pump shuts off, the tablets continue to leach in the skimmer basket, and start to overflow the skimmer. over time (a few years), this starts to fade and pucker the vinyl under the skimmer faceplate. Secondly, if using a chlorine floater, and not a chlorinator, the floater could become stuck on a step, or behind a ladder, or tip over and spill it’s contents, all of which could cause damage or fading to vinyl liners. And if you don’t run the pump all the time with a floater, the floater could sit in one spot for a long time, and bleach the vinyl on the wall.

      To avoid all of this, vinyl pools should use a chlorinator installed by the equipment. I like the Pentair 300 model, or the offline 320. Sani-King also makes a nice tablet feeder, and of course there’s Hayward chlorinators.

  26. Thank you for this great info! I was given a Hayward inline chlorine dispenser that says “use only tri-chlor”. I have not installed this yet. My fear is that if I only use tri-chlor in the dispenser, that my CYA level will become too high. FYI,I just installed a new liner, so the pool water is presently city water with one dose of cal-hypo shock. Thanks.

    • Davy Merino

      If your pool gets a normal amount of rain, or if you backwash frequently, or if you winterize the pool – these things help cyanuric acid levels from getting too high. Using trichlor tablets raises cyanuric acid levels, because there is a small amount in each tablet, but it can take years before the level rises to a level where it needs to be lowered, and mostly only in dry areas with little rain, and maybe a cartridge filter that is not backwashed, and a pool that does not have water lowered each fall for winterizing. All of these things dilute the water enough to keep it from rising too fast. But if it does get too high, you just drain and refill a portion of the pool, which is good to do every few years anyway – or now we have Bio-active, a product that reduces cyanuric acid in pools. So, I wouldnt worry about it getting too high, if I was you…

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