Pool Algae

Eradicating Algae From Your In-Ground Swimming Pool

Do you have extended travel plans this spring and summer? When you leave, one of your options for your in-ground swimming pool is to leave it covered while you are gone. However, when you return, the last thing you want to see is that your pool has turned into a green swamp. Getting rid of algae is a chore that even the most experienced pool owners have to face sometimes. With proper preparation, some purposeful attention for a couple of days, and a few minutes a day of follow up, you can eradicate the algae and turn your pool back into the welcoming sparkling oasis that was there before you left.

Your in-ground pool won’t turn into an unrecognizable green swamp overnight; however, we, at Hawaiian Pool Builders want you to have to tools and tips available should you experience this type of situation.

Algae is constantly growing. If you pause before you’ve gotten rid of it all, the guarantee is that it will bounce right back, and all the time, effort and chemicals will be wasted. Fighting algae is a race – you need to kill it faster than it grows back. To get rid of it on the first attempt, you need to hit it hard and follow up carefully to be sure it’s been eradicated.

Be Prepared
Before you begin the process, there are a few things you need to do, and know.

  • Your pump and filter need to be working. You’ll be letting the pump run 24/7 during the entire process to make sure the chlorine gets everywhere, or you might not get all of the algae. Make sure that you’ve cleaned/backwashed the filter. Also, know the filter’s pressure reading when it is clean.
  • You’ll need to know your CYA (Cyanuric Acid) level. This protects chlorine from sunlight and determines the required free-chlorine level.
  • You’ll need a chlorine test kit. It’s important to have a chlorine test kit available to measure the free-chlorine and the combined chlorine up to very high levels, which you need to ensure getting all of the algae from your pool.
  • Have plenty of chlorine on hand. The longer your in-ground swimming pool has not been clear, the more chlorine you may need. A good rule of thumb would be to have four times the amount needed to bring your pool to shock level.
  • Adjust the pH to between 7.2 and 7.4 prior to starting. The FC level will be high for several days, so you need to get the correct pH before you begin.

With your skimmer, make sure that there are no foreign objects in your pool. If your pool has sat for a while, you may not be able to see things that are in there, and sometimes items do get in under your cover.

Hit it Hard
Make sure you have plenty of time; preferably several hours in a row to work on your pool. You’ll also need to have time to follow up at least twice a day for the next several days. The more concentrated time you can spend, the quicker the entire process will be.

Set the pump/timer automation system to run continuously. You’ll be leaving it running until the water is completely clear. Now, you are ready to start killing the algae.

You’ll add enough chlorine to bring the pool to shock level. Wait about a half an hour, test the FC level, add enough chlorine to bring the FC back up to shock level, then repeat the entire process. Repeat this cycle until the FC level remains nearly at shock level after a one hour wait.

Once the green starts to fade, you should brush the entire pool. It’s important to brush everywhere while the FC level is high. This includes your ladders, steps and the entire pool surface. Algae can survive high chlorine levels if it’s formed a biofilm on a surface. By brushing, you disrupt the biofilm and expose the algae to the chlorine. Continue brushing once a day until the algae is dead.

Follow Up
To make sure you’ve gotten all of the algae, it’s important to maintain the pool at shock level until testing shows that it’s completely gone. Continue testing the water at least twice daily – morning and evening – and bringing it back up to shock level if necessary. Algae can hide in the most obscure places, like lighting niches, under a ladder, or in the plumbing. Continued high FC levels will kill it off eventually.

Additionally, brush the pool daily until you are sure it’s gone. Once most of the algae is dead, the water will turn milky or gray instead of green. It can still take the filter up to a week to clear the water from this point. A sand filter is the slowest to clear up the pool, while a DE filter can often do it in a couple of days.

While that is happening, you should keep an eye on the filter pressure and backwash/clean the filter any time the pressure goes up by 20-25% over your clean pressure. DE filters will often need to be backwashed a couple of times a day for the first day or two. Sand and cartridge filters will usually last a couple of days between cleanings. If in doubt, clean/backwash the filter more often rather than risk letting it get clogged up. If your filter pressure is rising quickly and you are not able to backwash as needed, switching to the recirculate mode, if available, on your filter’s multi-port valve can keep the water moving until you able to properly monitor your filter. Do not run through a solar panel system on recirculate or you could clog the tubes. If you have a salt-water generator, check that the cell also does not clog up with debris when running on Recirculate, remove the cell if possible.

Getting algae is sometimes unavoidable. Every pool and every situation is different. You can depend on the professional staff at Hawaiian Pool Builders to guide you through the process. Our retail store, located next door to our showroom, has all the supplies you will need to eradicate the algae from your pool, and turn it back into your sparkling oasis of summer enjoyment.


Pool & Hot Tub Alliance - Certified Advanced Pool Builder Professional
Pool Financing
Pool & Hot Tub Alliance - PHTA Member Anniversary 15 to 19 years