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 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, it’s 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 and 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 — 2 Comments

  1. 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