How to treat an Algae bloom

Every pool owner at one time or another will have to deal with the dreaded A-word: "Algae".

Algae is a small water borne organism that grows in your pool when your chlorine levels are too low or your water is not filtered properly. For many pool owners the presence of algae can cause stress and anxiety, because of the cost of chemicals and the amount of time needed to treat and clear up the algae bloom.

Removing algae from your pool is a process and there is no miracle chemical that will clear your pool in a few hours. Be sure to follow each step and not cheat, you are only hurting yourself and your pool if you do.

Step 1. Check And Clean Your Filter

The very first step when trying to clean up a green, algae infected pool, is making sure your filtration equipment is functioning properly and is clean. No matter what type of filter you own, the key to cleaning up algae is to run your filter for as long as possible. 24 hours a day is ideal if you have algae present.

If you are experiencing algae after taking the cover off the pool at the start of the season chances are you had algae when you closed. Below is a list of steps to follow for each of the 3 major types of pool filters:

  • Sand Filter

    Be sure to change your filter sand. It is common to hear filter sand does not need to be changed for 3 to 4 seasons. This is not true. Understand your filter is only running for 3 to 4 months out of the year. During this time the sand is constantly wet and waterlogged. The other 8 to 9 months the sand is sitting in storage wet, with no activity causing bacteria and mold to grow. When sand filter owners bring us their water to be tested, the most common statement we hear is: "I opened my pool and it was beautiful, and then it just changed on me." This is because the mold and bacteria in the filter has now been introduced into the pool water, causing an algae bloom.

  • Cartridge Filter

    Be sure to clean your cartridge thoroughly and soak it overnight in either a PH Decrease mixture or other non-invasive filter cleaner. The most effective method is to use PH Decrease in a 5:1 mixture with water (1lb of PH Decrease for 5 gallons of water). The average cartridge filter will last 3 to 5 seasons so a simple soak overnight in any of the solutions above will help open the pores of the filter. This way you will be ready to go for the season with a clean efficient filter.

  • Diatomaceous Earth Filter (aka DE)

    De filter owners will want to flush out their filters and start with new DE. Prior to hooking up the filter inspect all o-rings to make sure they are not dried out or cracked. You will also want to inspect the fingers or grids, depending on what model DE filter you have, to check for tears or holes in the fabric. This will prevent your filter from pushing DE powder back into the pool water.

Once again, no matter what style filter you own, please make sure that your water is circulating properly. Constant filtration and circulation is the key to killing off an algae bloom.

Step 2. Brush And Vacuum Your Pool Wall and Floor

After you have cleaned your filter and have it hooked up to the pool, the next step is to thoroughly vacuum your pool floor and remove any leaves and other debris from the bottom of the pool. Once you have removed all of the debris from the bottom of the pool, you want to thoroughly brush the walls and floor of the pool with a brush designed for your pool. The reason this is important is you want to cause the algae to become suspended in the pool water. When algae is floating in the water, as opposed to sticking to the liner, your filter has a better chance of removing the particles.

Step 3. Balance Your Water

Many pool owners start to see their pool water turn green or green splotches on the liner and will immediately start going crazy adding chlorine shock to clear up the algae. This should NEVER be your first option. In order for your chlorine shock to work well your balancing chemical levels have to be proper. This is especially true with your PH and Total Alkalinity levels. PH should always be between 7.2 and 7.8 and Total Alkalinity should be between 80 and 120 ppm. For more information on balancing your water be sure to check out our water chemistry and balancing guide. The presence of algae in the pool can cause these levels to become askew, so it is very important to test your water often.

If you are unsure about testing your water or would like a professional opinion please feel free to bring a sample of your pool water into any At Home Recreation location for a free state of the art computerized water evaluation. Lastly, remember as we stated above, your filter should be running the entire time you are adding chemicals and 24/7 when there is a presence of algae. We cannot stress enough how important filtration is when killing algae.

Step 4. Add An Algaecide Or Algae Sequestering Agent

Once you have finished brushing your pool and the water looks like a green cloud, you can add either an algaecide or algae sequestering agent. The most common chemical is algaecide, usually in a liquid form, the most powerful of which is 60% strength non foaming. A treatment of this algaecide is 4 to 6 oz per 10,000 gallons of water circulating with the filter for 6 to 8 hours. Another option that we have found works really well is an algae sequestering agent like Yellow Out or Green to Clean. These chemicals work in conjunction with powder shock to clear up the pool as quickly as possible. These chemicals are easy to use, as long as you follow the instructions on the packaging for optimum performance.

Step 5. Shock The Pool

Whether you are using an algaecide or sequestering agent, your next step is going to be to shock the pool. Again, remember that your filter should be circulating at all times during each step. The timing for this step will depend on which chemical you chose in step 4. If you choose algaecide, you are going to want to shock the pool 4 to 5 hours after adding the algaecide with either liquid or powder shock. If you choose the sequestering agent then you want to shock the pool within 15 minutes per the instructions with powder shock only. Your typical shock treatment equals 1 gallon or 1lb of shock for every 10,000 gallons of water. For the purposes of killing algae however, we suggest doubling that amount. Make sure you run your filter for at least 12 hours after shocking the pool.

Step. 6 Vacuum The Pool

After shocking the pool and running the filter for 12 hours you are going to want to vacuum the pool, usually the next day. You should already see a difference in your pool water. The water should either be cloudy blue or lighter green than before. When you vacuum the pool you want to make sure you are vacuuming to waste. If you have a cartridge filter and are unsure how to do this, you just have to hook up a backwash hose to the drain port on the tank. The water will then divert out the drain instead of back into the pool. By vacuuming to waste you are making sure all of the dead algae is being flushed out of the pool and not being recirculated back in.

Step 7. Re-shock The Pool And Backwash

After vacuuming it is always a good idea to re-shock the pool with a normal (1 gal/lb per 10,000 gallons of water) dose of shock. This gives the water one last kick to get rid of the algae that might remain. You also are going to want to backwash the filter or clean the cartridge depending on what filter you own. If you still see some traces of algae you are going to want to repeat steps 2 through 6 again.

As long as you follow these steps you will see your pool clear fairly quickly. If you have any questions about anything you have read here please contact us at

Previous article How to pick your perfect Christmas tree
Next article How do I open my swimming pool for the season?


Jesus Ibarra - January 13, 2022

Should I change the sand in pool filter after I cleaned out algae with shock? Like if the pool was completely green before, I cleaned it with shock and also self vacuumed. Should I change the sand in the filter although it seems to be running fine and water is now Chrystal clear? By the way it wasn’t sitting in algae that long I filled up the pool about a month ago, didn’t take care of it so it got full of algae. Cleaned it like I said, runs fine but is changing the sand recommended too?

Leave a comment

Comments must be approved before appearing

* Required fields