Shocking a Pool to Remove Algae

floating-algaeAfter what seems to have been an endless winter, the warm days can’t get here soon enough, and they’re finally right around the corner!

Now close your eyes and imagine… OK, so I guess you can’t close your eyes and read at the same time, so try to imagine with me… blooming flowers, brilliant blue sky, calming breeze, warm sunshine and a refreshing dip in your luxuriously beautiful… GREEN POOL!?!

Hopefully this won’t happen to you this summer, but if it does, here’s what you’ll need to know about shocking your pool to remove algae.brush-the-pool-

Before you start pouring shock in the pool, the very first step is to brush the sides and floor of your pool to loosen all the algae. Doing this breaks the ‘skin’, and allows the pool shock to more easily kill the algae.

ph-7.lowOnce you’ve done this, it is important to make sure you have the proper pH level in your water. The best pH level should be on the low side, between 7.2 – 7.4.

A high pH level can prevent the chlorine shock from properly killing the algae. Be sure to test your pH before adjusting, with a fresh and reliable test strip or test kit.

chlorine-30highIn order to clear a pool suffering from a severe algae bloom, a chlorine level of about 30ppm is preferable for optimal results. If you aren’t dealing with a full blown algae bloom, levels of 10-20ppm can be effective.

How much shock is needed to achieve 30ppm will vary depending on the available chlorine percentage of the shock you are using. pool-shock-treatment-chartFor example, if using In The Swim Super Shock, it’s a 73% cal hypo shock, but HTH Shock -N-Swim is only 45% cal hypo; so you’d use more – or less, depending on the shock potency.


bucket-o-waterThe best way to administer shock into your pool is by pouring it into a bucket of water with at least a couple gallons of water. Mix it to dissolve, and pour the mixture around the perimeter of the pool. Remember: always add shock to water, never add water to shock!

Now it’s time to wait a while. Keep your pump and filter running. Give the shock a good 12 to 24 hours to work it’s magic. If the algae hasn’t cleared up after 24-48 hours, clean and brush the pool and add another shock treatment.

When the chlorine has completely finished working, the algae in the pool will turn a white/gray color and will either settle to the bottom of the pool or be suspended in the water. There shouldn’t be any more green color and the water visibility should be improving. Run the filter 24/7, and backwash as needed.


When the water begins to clear, it’s time to vacuum your pool to get rid of all the dead algae. You can use a flocculant to settle suspended debris or a clarifier to phosflocaid in filtering. It is best to vacuum the dead algae to waste. You don’t want to trap all the dead algae in your filter. That can create a recurring algae problem.

Once the debris is gone from the water, it’s time to brush again with a new or good condition pool brush. Make sure there is no residual algae left on the walls or floor. Brush vigorously! If you feel the pain in your shoulders and arms – you’re doing it right!


After a thorough vacuuming and brushing, it is a good idea to clean your filter as well. If you have a DE or cartridge filter, remove the element and soak or spray with In The Swim Cartridge Filter Cleaner, filter-perfectand spray away all dirt and algae. Sand filters are a little more difficult to get clean. You can either replace the sand, or you can use a Sand Filter Cleaner. Natural Chemistry’s Filter Perfect does a great job on any filter type, using natural enzymes and citric acids.


Now your water is clear, the walls and floor are algae free, and your filter is cleaned of any dead algae – what’s next? Test your pool water again.

Test and adjust your pool to these levels: Free Chlorine: 1-3 ppm, pH: 7.2 – 7.6, Alkalinity: 80 – 120 ppm and Calcium Hardness: 200 – 400 ppm. Proper water balance in your pool is crucial to keeping clean water and staying algae free.

danger-zone-signAny time your pool drops below 1ppm of free chlorine, you are in the danger zone for algae (not to mention bacteria and viruses), especially if your pH and Alkalinity levels are off too! When your chlorine level is not high enough, it fails to kill off organic compounds that aid in algae growth.

Algae feeds off of phosphates found in plants and almost all other things in our environment. When phosphates enter the water with chlorine below 1ppm, you’re almost sure to have an algae bloom. Sometimes it can even happen in a matter of hours!


phos-freePrevent algae with with good water balance and constant chlorination. There are a lot of maintenance products out there to help safeguard against algae and keep it from ever being an issue. For extra algae prevention, use weekly maintenance doses of algaecide and Natural Chemistry’s PhosFree, to eliminate an algae food source.

Algae is a lot easier to prevent than to remove, but once you have it, brush the pool, lower your pH with a pH reducer and then Super Shock the pool. Vacuum and brush again and clean the filter – that’s the most effective way to get your pool back to normal.

So now that you’re back to a crystal clear, beautiful blue swimming pool, who’s ready for a dip?!



Christine Silvestri
InTheSwim Staff Blogger



Shocking a Pool to Remove Algae — 26 Comments

  1. Pingback: No Drain Pool Acid Wash | InTheSwim Pool Blog

  2. I am new to pool ownership. This is my first summer tuning it. I have a vinyl liner and have kept my chemicals perfect all summer (chlorine, Ph, and alkalinity), but it has been raining nearly nonstop for almost 2 weeks so algae got ahold of my pool. I shocked the pool 20k gal pool with 4lbs of shock with no change so I added 2 more lbs and that knocked it out. My Ph went from high reading to 7.2, chlorine is a 2, and alkalinity is 80. My pool is clear now, but cloudy. I’m waiting until morning to decide how it looks and what I may need to do next. Should I mess with the alkalinity and add 5lbs to bring it up a bit? I’ve never used clarifier, but it seems that might get rid of the milky/gray look to the water. Thanks!

    • Hi Sarah, the low pH level is good for shocking the pool, chlorine is much more active at a low pH level. Alkalinity is OK at 80 ppm. Clarifier is a good final step after shocking, after the chlorine level drops back down a bit, especially useful for pool filters that are super effective.

  3. Hi! Thank you for this advice and good information. We have a 25,000 gallon chlorine pool and I use a floating solar copper ionizer to help keep the algae at bay. I have a few random black dots on the bottom of my pool and on the steps on the opposite side. I can’t figure out if it is from the drops of copper that drained out and stained the pool or if it’s black algae. I have brushed, added 5-6 bags of shock, whole container of algaecide and run the pool for 36 hours straight. They’re still there.

    • Hi Abbey, it could be a part of the ionizer cell that fell apart, does it look kind of blue/green oxidized and flaky underneath the ionizer? That would likely be a copper sulfate, which could appear black. Black Algae, on the other hand, has a pronounced head that is slimy and raised 1/8″ or less. The heads can be scraped off with a butter knife or with a pool chlorine tablet. If there is no round slimy head, I don’t think its algae (you may have to inspect closely, with a mask or goggles). Sorry for all the chemical cost if so – for a metal stain, you can use a lb. of pH decreaser (or EZ Stain remover) in an old orphan sock (finally a use for those!), tie it off tight and cut off the excess. Use your pool brush to position over the stain for 30-60 seconds, and move around with brush, or get in there and scrub it with the stain sock. A Jandy Stain Master can also be used to deliver liquid acid directly to the source. If your entire pool has an overall staining however, trying to remove these small dots could produce the opposite stain, where now you have larger white dots on the pool 🙂 So, proceed slowly. Scrubbing with a pumice stone or PoolStone is also an option.

  4. My mother in law has a pool and refuses to let us wear the same swimsuits in the pool that we wear to the lake for fear of algae. Even if I wash them she’s still afraid they’ll contaminate the pool. After several years of hearing this I’m determined to prove her wrong. What’s your professional opinion? Is there any documentation or known occurrences of this being an issue?
    I’m just tired of having two sets of swimsuits for every member of my family.

    • Hi Angie, you think you’re going to win a fight with your mother in law! lol – but seriously, there may be some merit to her claim. Pools and lakes are very different and what may be a beneficial algae and bacteria in the lake, can be very much a problem in pools. So I have to say it’s probably a good idea, to use two suits (sorry).

  5. Other chemical company recommends higher PH of 7.8-8 when fighting algae that is established. You are indicating lower than normal.
    which is it or should one try both extremes to see which throws the algae out of its comfort zone?

    • Hi Phil, if you are fighting algae with Chlorine, you definitely want the pH to be on the low end of the scale for better chlorine efficacy and potency. If you are using a chemical which specifically states on the bottle to raise the pH to 7.8 or 8.0 for example, then follow that advice. Algae prefers high pH levels, so in most cases that I am aware, a lower pH will yield better results.

  6. Green slime, walls and bottom scrubbed, PH7, algaecide, and heavy shock on 15x30x4 above ground. 16 hours and no change, worried, miracle or new water?

    • Hi Sue, probably not enough shock added – keep adding until the water turns a blue/grey color. Could take double what you added. You’ll know if you add enough, when the water is still blue the following morning, and there is still a testable chlorine reading in the water. If it looks greenish and zero chlorine – you missed the mark 🙁 – try again. Also, shock will break apart algaecides, and algaecide doesn’t kill algae very well anyway (prevents). Raise the pH slightly to 7.2-ish, and shock the pool with 3, 5, 7 (?) lbs of shock, pre-dissolved into a bucket of water – keep adding until the color changes. You can also supplement granular shock with several (3-5) gallons of unscented bleach, if you want, added separately. Run the filter non-stop, and clean when pressure gauge rises and flow rate slows. After shock chlorine level drops, usually after a day or two, add clarifier to help the filter, and begin using algaecide. And keep the chlorine high with tablets. Draining a portion of the pool (up to half) and refilling, can also help reduce the solids level in the pool, and help the filter clean the pool faster, if that is possible for you. Also be sure the cyanuric acid level is 20-50 ppm, stabilizer to protect chlorine from the sun.

  7. Hello,

    I have a 20-year-old , 15,000 gallon pool with a LEGENDARY 1.5 HP DuraGlas PUMP, a 100 sq ft Posi – FLO FILTER and a Master Pools Turbo Clean infloor cleaning system. The inlet pipe to the pump is 2″ , and the pump outlet and filter inlet and outlet pipes are 1 1/2″ . I know there’s a tiny, hairline crack in the pump lid, which is probably introducing some air into the suction side, BUT the pump primes itself fairly rapidly (although the water seems to be about halfway down the pump pot basket when it’s started each morning), there is decent pressure though the water returns, and there’s good suction in the port below the skimmer basket. As such, I THINK the pump is performing at least adequately. Do you have anything ( e.g. two part epoxy) that repairs lid cracks??

    The FILTER is working, at least partially, as the FILTER element has been accumulating some green algae. There’s also debris in the pump basket strainer.

    Even though I run the pump 10+ hours a day, keep the water chemicals balanced ,and constantly Clean the FILTER element, pump and skimmer baskets, I’m STILL having algae issues; which is VERY FRUSTRATING!!!!

    I added fifteen (15) gallons of chlorine – A LOT of chlorine for a Pool that size – the other night and have been running the pump ever since , BUT the algae STILL hasn’t cleared. I performed the OCLT test yesterday, And the chlorine level stayed about the same, SO I THINK that’s a GOOD sign. Correct me if I’m mistaken, BUT I believe that even if there were poor circulation, 15 gallons of chlorine should still do a number on the algae, especially since I distributed the chlorine evenly around the pool. Perhaps the 100 sq ft filter just can’t remove the algae growth fast enough?? Any info you can provide would be appreciated. Thanks.

    • Hi Charlie, thanks for the detail. For the pump lid crack, you can use a number of sealants, but a clear silicone gel would likely be best, first clean the lid very well, inside and out, using soap and scrub sponge to remove oils and dirt. Then apply a bead of superglue into the larger part of the crack, let dry 20 mins. Reinstall the lid, start the pump and then apply a bead of clear silicone on the outside, and smooth with a putty knife somewhat. Leave the pump running for at least 8 hours or until full cure.

      For the algae issue, I would test the pool for phosphates, we have a AquaChek phosphate tester, or just treat with PhosFree. If you have not replaced the filter cartridge in many years, doing so may yield good results. Algae can harbor deep in the pleats of cartridges. 100 sq ft filter is OK, not great for your pool size, I normally like to see a little larger, so it may be overwhelmed at times. Using an algaecide regularly can help pools that seem prone to algae, but never add shock and algaecide together, the shock will kill (render useless) most types of pool algaecides, which are better used for preventing, not killing algae.

      For pools with algae struggles for years, with annual blooms for 5+ years, and getting worse each year, a radical treatment is to drain the pool and pressure wash, chlorine wash (or acid wash) (a concrete pool), replace the filters, nets, brushes and other soft surface items that have any potential algae spores, refill and start over. A larger filter wouldn’t hurt either, 200 sq ft, in your case.

    • I’ll bet your cynauric acid level is high.Had the same issue you need to drain pool water to get it to about 40-50 , mine was at 210. Do some research I guarantee that’s your problem. I was so Pissed nobody ever told me about. Faught it for 2 years

  8. Hey – I am hoping you can help. I have had my pool for five years and have been using Pristine Blue. I have never had any trouble. For the first time – this year at pool opening, I had Algae. I cleaned and shocked it. My pool changed to a pretty blue color (clear as drinking water when I pull out a sample), but I can not see the bottom. The pool store told me that I killed it, but it is still floating in the pool. I have followed all of my pool store instructions – for three weeks I have run the filter 24/7 (except when I used flock – I still could never see the floor of my pool) – I have brushed the pool – used the vacuum – backwashed the filter regularly – and I have used all the shock and chemicals that I purchased for the summer.
    Not many people in my area are familiar with Pristine Blue – the nearest dealer is two hours away – so having someone come out and look at the pool isn’t really an option.
    I am wiling to change over to a chlorine-based maintenance if necessary, but I am not for sure how to transfer from copper based to chlorine based.
    I would really just like to use the pool sometime this summer. Any suggestions?

    • I think to convert, you don’t need to do much, just start using chlorine tablets, in a floater or better in a chlorinator, and stop adding the Pristine blue. I would add a stain & scale chemical regularly, to keep the copper dissolved.

  9. So I had algae last year towards end of season got rid of it. Open the pool and was fine now all of a sudden it’s back. I’ve shock pool and vacuumed several times and still here. Filter was cleaned Getting so frustrated. Please help

    • Hi Jason, algae can be like that – it harbors until conditions are right to bloom again. Be sure that your pH and alkalinity levels are in range, chlorine works poorly in high pH conditions. Filter was cleaned? Algae can survive deep in the filter tank. If possible, replace your filter sand or filter cartridge (if you have DE filter, remove and soak entire grid assembly in a trash can with 1 gallon bleach to 20 gallons of water, remove and dunk in pool, or rinse well with hose). Also be confident that you are running the filter long enough each day, you may need more hours. Finally, you can buy some phosphate test strips, and test your water for phosphates, a yummy food for algae. If over 300 ppb, treat the pool with a phosphate remover. If you have shocked the pool but the algae persists, check that pH is kinda low 7.1-7.3, and then add a triple shock treatment (3x normal dose). If that doesn’t kill it, try a 5x shock (5 lb per 10K gallons). good luck!

  10. We have an 18 round pool with a sand filter we added 3 bags of shocker and a half bottle of algaecide and 2 tablet in the floater pool is still green I told my husband to brush the sides and bottom of the pool and he said there’s no need to do that

    • Hi Lori, well I hate to get in between you and your husband, but I would brush the pool, yeah. Also I would check the pH, make sure it’s 7.2-7.4, and then shock the pool again, 3-5 lbs or until it turns a blue grey color. Run your filter 24/7, and skim, vacuum and brush daily. Backwash when pressure gauges rises 7-9 lbs higher than clean pressure.

  11. Hi! We are new to caring for a pool. So upon opening our 18×48 above ground pool this is what we have done, cleaned out biggest part of leaves & larger debris. then got the pump & filter up & running. Added 128 oz of liquid shock & left for a little over 24h. Then we vacuumed the pool. My husband left the filter setting on filter instead of waste or backwash without thinking. Part way thru vacuuming the water went from clear to milky to where we could not see the bottom any longer. so he finished the best he could. The next day the pool was still milky & discolored with all the yuck from the water setting all winter long still visible somewhat. We added another 128 oz of shock & have left it filtering for several days & no improvement. We then added 16oz of clarifier & have left filtering for 2 more days. We can see the bottom somewhat now but the water is still far from clear. Can you please help us to know what we need to possibly do. Again I can not express how little we really understand about it all. Thanks!

    • Hi – sounds like you do know what youre doing, you’re just waiting for the water to clear. You didn’t mention pH and alkalinity, so I’ll ask about that, best to be in the 7.2-7.4 range. Otherwise, just let the filter run all day and night if possible, and backwash it only when needed. It will clear eventually. You can do another clarifier treatment, after 5-7 days from the first, but be careful not to overdose, only what the label says. Keep the chlorine level reading at 2-3 ppm, by checking it daily and adding chlorine tablets to a floater. Shock again if you see algae forming. Check pH at least once per week – it’ll come back, but with a small filter, it can take some time… 🙁

  12. I treated my green pool with shock and brushed before hand. Then when the pool started to clean up a little I brushed again and now it is green again. Do i need to reshock the pool or just let the filter run it’s course? I have a cartridge filter and clean them every day. Secondly I have a variable speed pump, is it better to run at high speed for turnover till clean or lower speeds to filter better?

    • Hi – check the water chemistry, if you have a good solid level of chlorine, and good pH level, then let it ride. If your chlorine level is close to zero, and you see traces of green algae, probably best to shock again. I would also run the pump on high, until you get clean and clear water again. Cartridges should not need to be cleaned daily, but maybe, if the filter is very small, or the cartridges are very old.

  13. I need to run that bad algae thru my filter when u put shock in ? I was thinking I needed to backwash on waste using m leaf net to get leaves out left over. Its not many now but I need to know it’s ok to run all that ukky thick algae thru my filter

    • Hi Doris – Yes, you can run it through the filter, depending on the size of your filter it will clog up sooner or later, but eventually will need cleaning. You can vacuum to waste if you have multiport valve, that is a good idea, but running the filter is important to remove the dead algae from the water. Keep an eye on the filter pressure and water flow rate, and clean or backwash the filter as needed to maintain both

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