Welcome back to school, students! Summer 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 to eradicate algae blooms – powerful and economical.

Our discussion will focus on how much pool shock to use for complete algae removal, 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 as much of the debris as you can 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.The dose listed on a bag of shock (1 lb. per 10K gals) is really just for clean and clear water, for algae, you need much more! What is 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 12-18 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, a triple shock is needed. 1 bag will get reach 7-9 ppm, but for 30 ppm, you need 3, 4 or sometimes even 5+ lbs per 10,000 gallons of pool water.

“Shock it until the water turns Blue-ish”, said Davy Merino, our blog editor. “That’s what I do”. A fine technique – you do want to add shock to the pool until the water loses the green color, and turns a blue-ish 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.


How much shock is needed to achieve 30 ppm will vary depending on the available chlorine percentage of the shock you are using. For 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.

8 Tips for More Effective Algae Removal

  1. Put away the algaecide – pool shock destroys or deactivates algaecide.
  2. Lower the pH before shocking, 7.2 – 7.4 is best for shock efficacy.
  3. Dilute pool shock in a bucket of water for vinyl liner pools.
  4. Run the filter 24/7 until water is clear. Backwash only as needed.
  5. Brush the pool vigorously, several times after shocking the pool.
  6. Do not use a solar blanket until chlorine and pH level are normal.
  7. If chlorine level drops to zero within 24 hours, Repeat the shock treatment.
  8. Improve filtration with a pool filter cleaner or filter aid like Jack’s Filter Fiber.


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. Using a Clarifier can be helpful to help speed the filtering of small particles; shock dust and dead algae.

If possible, 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. Vacuuming to Waste is hard with a cartridge filter, but not impossible.

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!

pool brush


filter-perfectAfter 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, and 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 maintaining clean water and staying algae free.

danger-zone-signAny time your pool drops below 1 ppm 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 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 1 ppm, 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.


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 filter sand or a filter cartridge replacement. In the case of Yellow Algae, Pink Algae, or White Water Mold, filter media replacement is strongly recommended.

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

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

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

charlie-oserkis-pool-southern-new-jerseyKeep America Blue!


Dr. Pool