Spa Shock – How to Shock a Spa or Hot Tub

dr-pool-lightning-boltWelcome back spa and hot tub enthusiasts! We have a shocking topic for discussion today, How to Shock a Spa or Hot Tub.

The term “shock” is something of a misnomer, although you may envision the oxidation process as an electric shock ripping through the water.

Shocking your spa effectively, for the right reasons, and without detrimental side effects, is the purpose of today’s lecture.

Don your lab coat and your protective eye wear as we begin today’s chemical lab to better understand oxidation of hot tub contaminants.

What really happens when you shock a spa? Oxidation is the process of a particle losing electrons when it encounters a disinfectant.

oxidation-2In the case of granular chlorine or non-chlorine oxidizerspa shocks, the oxidizer rapidly steals the outer electrons of contaminants it comes in contact with. The loss of these electrons makes a contaminant vulnerable to attack at a cellular level which quickly destroys the molecule.

Reasons for Shocking a Spa or Hot Tub

fourThere are only four valid reasons for shocking your spa or hot tub. It’s not necessary to shock the spa every time you use it and maybe not even weekly. Shock your spa for these four reasons:

  1. To remove bromamines or chloramines in the spa. Using a DPD test kit, you can test for the level of combined chlorine or bromine. Adding an oxidizer in the amount equal to 10x the level of combined sanitizer will break apart these bonds.
  2. To remove organic contaminants in the spa. When 3 or 4 users jump into a hot spa, the level of contaminants spikes to a level many times higher than your sanitizer can manage. Adding an oxidizer afterwards will restore sanitary spa water.
  3. To reactivate bromides in your spa water. For a bromine spa, using an oxidizer weekly helps to maintain a consistent bromine bank in your spa water. Activating bromine ions with spa shock converts them to hypobromous acid, the killing form of bromine.
  4. To Kill bacteria. Remember that non-chlorine shock is not classified as a disinfectant like chlorine or bromine, but is useful only as an oxidizer. Use chlorine spa shock to kill all bacteria and pathogens.

Preparing to Shock a Spa or Hot Tub

The most important thing to do in preparation for shocking the spa water is to first test the spa, and adjust the pH to the range of 7.2-7.6. Oxidizers work best in a low pH environment, and the efficacy of your spa shock is twice as great at a pH of 7.6 than it is at a pH of 8.0.

spa-cover-removal-before-shockingThe second most important thing is to remove your spa cover, spa blanket and spa users before shocking the hot tub. Especially during the first 20 minutes, your spa surface needs to be aerated to facilitate the exchange of electrons and gassing off of byproducts formed during oxidation.

The third most important thing is calm water – don’t turn on the blower but do run the circulation pump when you shock the spa. Too much agitation can prematurely gas off potent oxidants.

Shocking a Spa or Hot Tub

When shocking the spa for the above reasons only, pay attention to a few things. Specifically, be cautious when using spa shock, oxidizers can be dangerous when misused or mishandled.

  1. Read the Label – different oxidizers have varying strengths. Be careful not to overdose or underdose your spa shock treatment.
  2. Measure your spa shock – dosing is so important. If you are 1/2 of an ounce off, you can underdose or overdose your spa.
  3. Add Carefully – high winds can blow spa shock right in your face. Gently pour it over the surface and wash away any spills right away. Keep out of the reach of

Which spa shock should you use? Use an oxidizer specifically labeled for use with spas and hot tubs. Don’t use bagged pool shock in your spa.

We have a large variety of spa shock that can be used for oxidation. Choose one you like and get on with it!

Remember my cautions above when using spa shock, because – Oxidants Happen! Sorry, couldn’t resist…



Dr. Pool



Spa Shock – How to Shock a Spa or Hot Tub — 28 Comments

  1. Our water becomes very cloudy after heavy usage (20+ people) in one day. We shock but water still remains cloudy. We did a full drain and refill less than 2 weeks ago. Should we do something different than 2 tablespoons of shock? It doesn’t seem to clear the water.

    • Hi Michael, yes I would shock it with a triple dosage after entertaining so many people. Or give the spa hour breaks, and shock a few times during the day, and let the filter catch up. Also check the pH level daily, as many people can throw it out of whack. And, your spa needs extra filtering, so be sure it’s running all the time for extra filtering. Change out the filter cartridge more often and change the water more often too. A supplemental sanitizer should also be used, like ozone or minerals. Enzymes and clarifiers can help too. Even if you have an 8-person spa, some systems can become overwhelmed with just 4-people daily, or everyday usage. Spa usage of 20+ people per day requires serious sanitation, filtration and possibly changing the water weekly, if not daily – to keep it safe and healthy. Follow commercial spa guidelines and be safe! you don’t want your guests to develops rashes and other infections…

  2. I wonder if you would recommend to change 1/4 of the water in the hot tub monthly instead of completely change the water every 3-4 months? This is in a rental unit.

    • Hi there – not a bad idea actually, and I know some folks that do it that way. In fact, it may be better to drain and refill incrementally, rather than waiting until the water really needs a full replacement. However, be prepared for those times when a complete replacement of the water is needed, for times when heavily used, or contaminated or power outage, etc. For most rentals, changing the water monthly (completely) may be needed, depends on how much the spa is used (or abused I should say). Use your judgement.

  3. I am extremely confused with treating my spa water. This is the first time in a few years I have used my hot tub. Here’s where I’m at: hot tub was drained and cleaned, then filled and a spa purge product used. After using the spa purge product I drained that water. Again cleaned out the hot tub. Yesterday I filled with new water and first used “Spa Stain and Scale”. After allowing that to work I used “Spa Metal Free”. Today, I tested my water and my pH was a bit high, but my alkalinity was a bit low. I used “alkalinity up” and let that work. Upon retesting, the alkalinity and the pH were predictably both high, so I then used leslie’s “Dry Acid” to lower pH and Alkalinity. Once my alkalinity and pH are in normal range I am confused on where to go from there. Should I put chlorine in as the next step or do an oxidizing shock? No Chlorine has been added at all yet and is chlorine is showing up white or absent on my test strip. Please help me! I’m so lost!

    • Hi Jordan, you are on the right track! Stain & Scale and Metal Free are used to lock up minerals and metals in solution, for spas on well water, and especially for plaster inground spas which stain more easily. If you don’t have well water, and don’t have a plaster (cementitious) finish, and don’t have a metal problem, you can skip it next time. And, both products do the same thing, so you don’t need both I wouldn’t think. For pH and Alkalinity, as you know both are related and adjusting one affects the other, so sometimes you have to see-saw with pH and alkalinity chemicals until both come into range (80-120 ppm for Alk, 7.2-7.6 for pH). Now for the sanitizer, chlorine or bromine. I would shock the spa first, using either a non-chlorine shock or chlorine shock, either one. Shocking is used weekly or so, depending on the spa usage, to remove bacteria, break down organics and other undesirable compounds. But you also need a Daily sanitizer, either the 1″ chlorine tablets in a small floater, or the 1″ bromine tablets, also in a small floater. use enough tablets to get a consistent and constant reading of 2-3 ppm of chlorine, or 3-5 ppm of bromine*. If using bromine tablets, there is one additional step, after draining the spa, shake in an ounce of Bromide Booster, to build up the level of bromide ions in the water, otherwise it can take weeks to begin to register a bromine reading. *If you also have a Mineral Stick (by Nature2, Leisure Time or Spa Frog), or if your spa has an ozonator, you can cut back your tablets by half, to run at 1-2 ppm chlorine, or 2-3 ppm bromine. Hope that helps!

    • Hi Glen, in a bromine treated pool (or hot tub), shock is used for the normal reasons (removing bacteria, oxidizing organics, removing bromamines), but the larger reason is that using a shock oxidizer (chlorine shock or non-chlorine shock), will re-activate bromide ions into bromine, in a never ending cycle. Active bromine reverts to bromide after it has done it’s job, and the shock changes it back to bromine again. Another part of using bromine in a spa, is that it takes several weeks of using bromine tablets, before the bromide ion ‘bank’ builds up, so many spa owners add an ounce of Bromide Booster to the water after draining/refilling, to re-build the bank instantly. So… when using bromine tablets, you also need to shock every week or two, and after draining, use a bit of bromide booster. Thanks, good question!

    • Hi, I like to use for such precise measures. For 320 gallons, using Cal Hypo shock, you would use 2 oz. to shock to 30 ppm, for severe algae. If the water looks good, and not completely green, 1 oz or even 1/2 oz. would be good. Basically a tablespoon of 65% cal hypo shock. IF you are using other types of chlorine (with different % of available chlorine), or non-chlorine shock, the amount would be different.

  4. My Sundance Marin states to turn on jet when adding PH balancing and shock why is this different then the recommendations stated above.TY

    • Hi, I see that the Dr. suggests turning off the blower and jet pump during shocking, to avoid the shock gassing off too quickly. pH adjustment it doesn’t matter so much, but alkalinity adjustment will favor calm water, for a more rapid exchange of hydrocarbons. Sundance would advise to have the jet pump running, to more quickly mix the chemicals, but more importantly, to make sure that large quantities of chemicals aren’t sitting directly on spa surfaces, undissolved, which could dull or harm the finish.

  5. Can I use liquid chlorine to shock in my 500 gallon spa w bromine tablets floating? What is the ppm I should hold daily? What should I shock up to (ppm) and how often when using 3-4 times a week.

    • Hi, yes you can use liquid, as a shock oxidizer, and also to reactivate bromides into bromine. For a 500 gallon spa use 6 oz of 5% bleach, to raise CL levels to about 5 ppm. For daily sanitation, keep bromine levels around 3 ppm, and shock the spa every week with about 3/4 cup of bleach, and see how that goes? You can also use non-chlorine shock after heavy use of the spa.

  6. We have just cleaned and refilled the spa, I have used the spa shock granules as measured and am running the spa on low
    Why do the granules just sit on the bottom or will they disintegrate?

    • Hi Jay, generally speaking, most MPS (non-chlorine) spa shock will dissolve instantly. Chlorine shock will take longer to dissolve completely, and can be pre-dissolved into a clean bucket or pitcher of water, and then added (always add shock to water, not water to shock). Also, spa shock that has been exposed to moisture may take longer to dissolve, and may have lost some of it’s power. For granules that sit on the bottom, in most cases they won’t harm the surface, but to reduce the risk, use a brush or net to stir them around.

    • Hi Mo, not before draining, unless you have a spa that has turned green or become stagnant. And not after filling, unless you suspect that the fill water may be unsanitary – which could happen. Shocking the spa is done for these reasons 1). To kill invisible bacteria and other pathogens that escape normal daily sanitation, and 2). To kill visible algae, mold or bio-film on surfaces, and 3). To break down contaminants like soaps, lotions, hair products, and 4). To remove combined chlorine (chloramines) and combined bromine (Bromamines), and 5). To reactivate bromide ions into active free bromine.

      Generally, most spa owners shock the spa every week or two, usually after exiting the spa. Follow label instructions, always use spa shock (not pool shock), and leave the cover off the spa for 30-60 mins afterwards. A good pH level of 7.2-7.6 is helpful to allow the shock to be most powerful.

    • Hi Ryan….generally my hot tub is not in use from Monday till Thursday, can I give it a shock on the Thursday morning and how long after shock can it be used?
      I have a 1500 Ltr tub, what measure of shock should I use?
      Will the shock raise the bromine level???
      Thanks for your help Ryan.

      • Hi, how much shock you should use in a spa, depends on the purpose of the shock, and what type of shock you are using, and of course how much water is in the spa. It also matters how much the spa is used 1-2x per week, or 3-4x per week – because the more it’s used, and the more people that use it, the more contaminants (from us!) that the shock needs to oxidize. For just 1-2 people, once or twice per week, the contaminants should not be out of control, so a normal dosage can be used, once per week. Assuming that you don’t notice any adverse water conditions (smell, color, cloudy, algae, foam), a normal dosage is also indicated. For spas with many people using it, or with noticeable water problems, or if power was shut off for a few days or for pump/filter repairs – then you can increase the dosage of spa shock, up to double or triple the amount shown on the label. For heavy use spas, once weekly shocking may not be enough, some spas need it almost daily. Good filtration and water balance is also important, and can affect the amount and frequency of spa shock needed.

        For your spa, since you don’t it that much, a weekly shock is fine (assuming that your filtering all week long, and maintaining a bromine level). Non-chlorine shock may state that you can use the spa immediately after treatment, but I’d wait 20 minutes, to let it do its thing without introducing more contaminants (you!).

        Adding shock to a bromine spa will increase the bromine level, yes. That is, if you have a good ‘bromine bank’, or build-up of bromide ions in the water. Aside from bacteria and bromamine removal, one of spa shock’s main roles in re-activation of bromide into bromine. When shocking a spa, some of the shock is destroying contaminants, and some is boosting the bromine level, and a heavy shock can raise your bromine level very high for awhile.

        For your 1500L (400 gal) hot tub, consult your dosage on the label, but it’s probably 3-4 tablespoons, or 1-2 ounces (28-57 gm) per treatment. Your own experience is a good tool to use, knowing the spa, and how it responds to certain dosages (given good water conditions and water balance), and will help guide your hand in treatment.

  7. How long should you wait after the shock treatment? If the water is very cloudy, Is it bad to do both a non-chorine shock, wait, and then do a chlorine shock? How long should I wait in between ?


    • Hi, you could use both, but i would probably wait about an hour between adding. Just be careful not to overdose, on either types of shock. Keep the cover open after shocking, and be sure not to mix oxidizers (or other pool chemicals)…

    • Hi, I like to use for such questions. You can type in gallons at the top, then set your current and target FC (Free Chlorine levels), then select your shock type from the drop down menu and… in your case, if you wanted to shock it good, to 10 ppm, you could use 0.5oz of DiChlor shock, or 0.4oz of Cal Hypo 65% available shock. If using non-chlorine shock, also about 1/2 oz.

      However, always consult the label on the bottle for specific treatment and dosage guidelines. Your pH levels should be proper for best shock effect. When shocking a spa, turn on the jet pump, and leave the covers off, to allow chlorine to gas off (and not harm your spa cover). Wait to use the spa until chlorine drops below 5 ppm. If using non-chlorine MPS shock, you can recover it or use it almost immediately.

    • I have a great formula for you – for 400 gallons, add 0.1 oz for every 1.0 ppm of chlorine you want. As you can see it’s very little. If you just want a little 1 ppm boost, add a teaspoon, or 1/10 of an ounce. Add a tablespoon full for a 4ppm superchlorination or 2-3 tablespoons for poor water conditions or algae. It doesn’t take much, use very little! By the way, we don’t recommend using pool shock for spas. Spa shock is much finer, suited for hot water, labeled correctly…The larger problem is that opened 1 lb bags of pool shock can be dangerous, the can spill, become contaminated or become wet – from open/closing the bag repeatedly, like you would for a spa.

    • Hi Mary Anne, the reason we don’t recommend bagged shock for spas is because storing opened bags of shock can be hazardous, from spills, moisture and humidity or contamination. It also tends to be slow dissolving, or not as fast as shock made specifically for spas. Also, granular chlorine should not be used in containers or with scoops that have contacted other chemicals. The smallest amount of residue is enough to cause a reaction, including explosions and fire.

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