Pool Shock: Shocking for Algae Removal


Welcome back to school, students! August is the perfect time to discuss algae and how to remove it by shocking the pool with Calcium Hypochlorite – Ca(OCl)2.

Chlorine is a Great Algaecide, I like to say. Faster kill rates and a more complete reaction than using algaecide or other oxidizers.

Cal Hypo is the perfect chlorine type to use as pool shock – powerful and economical.

Our discussion will focus on how much pool shock to use for a complete algae removal treatment, plus other tips on effective pool shocking.

Preparation for Shocking

1. Clean Pool Thoroughly. If your pool has debris or leaves on the bottom or floating on top, this will interfere with the process. Chlorine will attack this organic matter, rather than the algae.

If the pool is too dark green to see the bottom, use a Leaf Rake, or the Leaf Gulper and clean 98% of the debris from the pool before shocking the pool.

2. Lower the pH. Cal Hypo (and in fact all chlorine products) have greater efficacy at a lower pH level. Add an acid to adjust the pH to 7.2. Check your Alkalinity first, to make sure that it’s in the range of 80-120 ppm, before adjusting your pH level.

3. Assess the Algae. Determine the level of severity of this algae attack. Is this a summer bloom that was caught early, or has the pool sat neglected for months? Choose one of the three icons below to indicate the level of algae in your pool.


LIGHT GREEN: Water is mostly clear, with areas of visible algae.
MEDIUM GREEN: Algae in all areas, visibility low at 18-36 inches deep.
DARK GREEN: Thick algae deposits, visibility limited to 12 inches deep.

Shocking for Algae Removal

It’s a generally accepted notion that 30 ppm of chlorine residual will kill all algae. Assuming that your pool is clean, and you have adjusted the pH, you can now add the granular chlorine to the pool.

Most pool shock packages will list directions to add 1 lb. (1 bag) of shock per 10,000 gallons of pool water. That may be fine for normal conditions, but if you have a severe algae attack, more will be needed.

“Shock it until the water turns Blue-ish”, said one of our pool guys on staff here. “That’s what I do”. Not a bad technique – you do want to add shock to the pool until the water loses the green color, and turns a blue-gray color.


Here’s a chart that can be used with our Pool Shock and Super Pool Shock products. The numbers indicate the amount of shock, in pounds. You may need more to see the blue color begin to return to the pool.

5 Tips for More Effective Algae Removal

  1. Don’t use an algaecide before shocking, pool shock will break apart the polymer chains, rendering it useless.
  2. Check your pH, before shocking, and then again a few hours after shocking.
  3. Dilute your chlorine in a bucket for vinyl pools; for plaster pools, just pour it in.
  4. Run the filter 24/7 until water is clear. Backwash only as needed.
  5. Brush the pool vigorously, after shocking the pool.

Cleaning Up

After a few days, you can vacuum up the shock dust, or brush it daily and run the pool cleaner if you don’t have a way to manually vacuum the pool.

Brushing is an important step immediately after shocking and for a few days afterwards. Use a good quality pool brush for the best results.

pool brush

Using a pool filter cleaner after a major clean-up is a good idea. Depending on the age, you may also elect to change the filter media, with new sand or a filter cartridge replacement.

Once you have things under control, you can begin using maintenance doses of a good quality algaecide, maintain a constant chlorine level, and operate the filter for 12 hrs +/- daily.

In Sum, shocking for algae removal requires a good pH, a good brush, and a good dose of Cal Hypo pool shock, available in bags and buckets.

Keep America Blue!



Dr. Pool



Pool Shock: Shocking for Algae Removal — 19 Comments

  1. I have a in ground pool filled with well water. What would you suggest to kill dark green algae? I have been working on the pool for a year now. I have been removing the leaves and mud off the bottom of the pool, making progress. I inherit the pool and it hasn’t had care for at least 10 years or more.

    • Hi Maxine, draining and refilling the pool with fresh water, after a thorough cleaning and acid washing (cement pools only) is my best recommendation. If draining is not possible, then a very heavy dose of chlorine, with a pH level in the 6.8-7.2 range is the next best step, after removing all of the organics (solids, leaves and mud). However, there will be stains, lots of stains, and the water – is so choked with solids, that fresh water is really the best long term solution, to avoid continued algae and water quality problems. Also, be sure the pump and filter is operating properly and ready to go for the fresh water. The filter media (sand or cartridges) likely need to be replaced also… good luck – let me know if there are additional questions!

    • Hi Sherri, the main concern with shock is that undissolved granules may get into your eyes or ears, or irritate skin, esp if it gets stuck in between your suit – ouch! High levels of chlorine can also fade swimsuits and lighten hair. Your test kit could tell you – when it drops below 5 ppm, and granules are dissolved, it’s ok to swim. At 10 ppm, you may fade out your favorite swim suit, or experience skin irritation. In most cases, this is 24 hours, but you added quite a bit of chlorine, may take 48-72 hrs to drop…

  2. Hi I have a 20k gallon pool. Slight algae showing up on sides. Tested the water and ph is 7.2, free chlorine is at 0, alkalinity is at 70, phosphates are at 750 ppm. How much chlorine should i add. I would prefer liquid chlorine and have 2 gallons of 10%. Water is clear still. I just want to get my chlorine back to 2.0-3.0, my ph back up to 7.4 and my phosphates back below 100ppm. Thanks for the assistance.

    • I would first lower the phosphates, with a phosphate remover chemical, well actually, you better get some chlorine in there fast. 1/2 gallon should raise the chlorine to above 2.0 ppm, but the phosphates will consume some – then treat with phosfree or other remover, and then add more chlorine. Liquid chlorine produces the same hypochlorous acid as tablets, but it is hard to treat consistently – unless you are using a liquid chlorine feeder ($$$). If you want to stick with liquid (instead of tablets), it probably should be added at least once daily, to avoid peaks and valleys in chlorination. Also – try to find the source of phosphates if possible (fertilizer, mulch, soil…)

      • I have been using tablets in a floater and been able to maintain my chlorine level at 2.0 ppm all summer till now. We did go out of town and came home to an empty floater though. Im guessing the phosphates are coming from the leaves from our 2 shade trees and the dust that gets blown around regularly here in Arizona during monsoon season. I added slightly over half a gallon and will test it later today. Should i clean cartridge filters before and after adding phosfree or just after?

        • Josh, I’d suggest cleaning them first, if the pressure is 5+ psi higher than normal, if normal wait until 48 hours after treatment, unless the product label advises differently.

  3. Hi
    I just have a kids pool….intex 15ft x 33 inches round.
    I use the chlorine tablets….but now water is turning green.
    How much shock do I use?
    I bought a 1 lb bag.
    I think it is 2600 gallons of water

    Thanks for any helpappreciate it
    DIANE 😊 .

    • Hi Diane, use 1/2 lb, and then seal up the bag tightly inside of a ziploc, to keep out moisture and prevent spills. Before adding, check and adjust pH if needed, to be in the low end of the range, around 7.2-7.3. Predissolve the shock into a 5 gal. bucket of water, then pour around the pool, then brush pool, and run filter for at least 4 hours, if not overnight.

  4. I opened my pool about a week ago. It’s still green. I’m running my DE filter 12 hrs a day. I have a 27,000 gallon pool.. Help!! How much shock should I add. Should I dilute it or juse sprinkle it in

    • Hi, if it’s still green, you need more shock – keep adding until the water turns a blue/gray color, then add one more. For your pool, that should be 10 or 11 lbs of shock, to reach 30 ppm. You can pour it right into the pool if you have a concrete pool, vinyl pools should predissolve. Make sure your pH level is low, around 7.2, for best results or potency from the shock. Run the filter 22 hours per day, until clear.

    • Super shock is a Cal Hypo product so it should always be diluted by pouring into a full 5-gal bucket of water, stirring to dissolve and then pouring around the edge. If any granules fall in the pool, use a pool brush to push them around. It is the strongest shock we sell also, so be careful not to overdose the pool. I think I would recommend just regular pool shock, and not super pool shock, 65%, not 73% – for a vinyl pool, just to protect the colors of the vinyl.

  5. Hi, this if the first time we are opening a pool. I tried to get my husband to hire someone, but he insists on doing it himself. YouTube told him he could. Our pool is “dark green”, 25000 and the deepest end is 8 ft. The deep end is so dark it looks “hunter green”. We’ve been running the pump for 2 days now and he added the chlorine and two bags of shock (which is what the bag said we needed for our size pool). We also ran the pool vacuum all day yesterday. There has been no change. Three questions, how long after adding the shock should we see a change, should we have used more than two bags of shock and it said to clean the pool first, but the bottom is so dirty we can’t tell if there is anything down there other than algae, if there is, will it undo what we have done with the shock?

    • Hi Gwen, the two bags of shock didn’t do much, and were wasted unfortunately. The pool just ‘ate it up’. Here’s what to do. First make sure that the pH and alkalinity levels are good, best range is 7.2-7.4 for pH and 80-100 for alkalinity. Then, shock the *&^% out of the pool, with about 6 to 10 lbs of pool shock. Just keep adding until the water turns blue/grey color. Run the filter all night. In the morning, clean the filter and recheck pH and chlorine levels. If chlorine is at zero, but pool is still blue-ish and cloudy, add 2-3 lbs of shock. If it is zero but has turned green again, add another 5-6 lbs of shock. (I know it’s a lot, but trust me you need it!). Keep pH balanced, run filter for 24/7, add a clarifier a few days after the final shock. You’ll get there! I applaud your DIY spirit!

  6. My pool store told me to add 40 pounds of your shock to my 25000 gal pool for a no chlorine condition. Have you ever heard of having to add such a huge amount?

    • Hi, not for a no-chlorine condition. If your pool has very bad algae, I mean very bad – it may take 40 lbs, but if the water is clear, you can add just 1 lb, to raise the chlorine level again. A normal shock treatment is generally 1 lb per 10000 gallons of pool water, or about 3 lbs for your pool. For visible algae, 5-6 lbs may be called for, and for very bad algae or ‘tea’ colored water, up to 10-12 lbs is not unusual – but 40 lbs? They might be confused…

    • Ask them to check your stabilizer as well. That can cause the chlorine to not release. Just a thought good luck!

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