Swimming Pool Blog

Swimming Pool Algae: Death by Chlorine


Earth Day: How to make your pool more eco-friendly
by Guest Blogger Matt Giovanisci, April 25, 2012

picture close up of a green pool algae cellThe worst thing that could ever happen to your pool just did - Algae! What are you going to do? How are you going to stop it? More importantly, how are you going to KILL it?

Sounds harsh, but the truth is, algae can ruin a summer afternoon by the pool (maybe even several afternoons). You don’t own a pool so that you spend your free time getting rid of algae. You bought it because you want to swim in it!

No matter what type of algae has decided to make residence in your beautiful swimming pool, it must be destroyed, and die a thousand deaths with CHLORINE!!!


Chlorine is the ultimate algae killer. It kills:

  • Standard green algae
  • Stubborn yellow (or mustard) algae
  • Even the dreaded black algae.

With that said, you must first determine what type of algae you have.

Standard Green Algae

This type of algae is the most common. It’s green, it’s slimy, and it grows extremely fast in the right conditions. Conditions such as warm water, with high pH levels, and perhaps high phosphate levels. Tends to like the shady side of the pool.

Stubborn Yellow (or Mustard Algae)

Yellow algae is sometimes mistaken for sand or pollen, but if it’s attached to your pool walls or floor and easy brushes off and dissipates into the water, then it’s yellow algae. It’s stubborn because it has a resistance to chlorine and tends to grow back in the same spot if you don’t treat it correctly.

Yellow algae also travels well. It can travel on your bathing suits, floats, and maintenance equipment. All of these must be treated as well as the pool water.

The Dreaded Black Algae

Black algae looks like black spots on your pool wall or floor. We dread them because they have strong roots that grow deep into the surface of your pool, and much like a wart, it can grow back. It’s very hard to get rid of because it requires a good amount of brushing to knock the heads off, so the chlorine can penetrate into the roots.

Black algae, much like yellow algae, travels very well and most likely came from an ocean or a lake. Keep in mind that if you use the same bathing suit or floats in the pool as you do on the beach, make sure you wash and disinfect them before introducing them back into your swimming pool.

How To Kill The Algae

As I mentioned before, chlorine is your best weapon against algae. More specifically, you are going to use calcium hypochlorite pool shock to super-chlorinate your water.

First, let’s determine just how much algae is in your pool. If your pool is very light green or you see very little yellow or black algae, you will need to double shock your pool. That means, using 2 pounds of shock for every 10,000 gallons of water your pool holds.

If you pool is dark green or you see heavy spots of yellow or black algae, you will need to triple shock your pool. 3 pounds for every 10,000 gallons of water.

Finally, if your pool is black or really infested with yellow or black algae, you’ll need to quadruple shock your pool. 4 pounds for every 10,000 gallons.

Note: If your pool is really infested with algae, you might have to repeat the shocking process again the following night to completely destroy the algae.

Here are the steps you should take to kill the algae...

1. Remove all floats from the pool, gather up all cleaning equipment, and sanitize them using a chlorine or bleach solution. Also make sure you thoroughly wash and dry all bathing suits in hot soapy water.

2. Get your water tested to make sure you have the proper pH and alkalinity levels, and that your pool can hold chlorine.

3. Wait until the late afternoon or dusk to brush the entire pool. Paying close attention to hard-to-reach places like under the ladders and creases.

4. Turn on your filter and keep it running 24/7.

5. Shock your pool with the correct amount of shock as we mentioned earlier. Pre-dissolve each pound of shock in a bucket of pool water before broadcasting it throughout your pool. Make sure you cover the entire pool with shock as best you can.

6. Let your pool run overnight. Shocking at night helps the shock to do a more efficient job. Sunlight can eat up 1ppm (part per million) of unstabilized chlorine (like shock) every hour.

7. Keep brushing your pool for the next couple of days and brush often. Make sure you keep a good level of pH, alkalinity, and chlorine during this time. It’s crucial.

After a couple of days, if the algae hasn’t returned, you’re probably good. Keep brushing for good measure and keep a very close eye on your chemical levels to ensure the algae doesn’t make a return.

Finishing Up

At this point, your pool might be a little cloudy. It’s ok. It’s partly dead algae that is causing this. Your filter can do the job of cleaning up the mess you left on the battlefield. Just keep your pump and filter running until your pool is clear. You can add a little clarifier to help speed up the process. Backwash filter very thoroughly when pressure rises.

DE filters should have the grids removed and hosed clean after backwashing. Soaking the grids or filter cartridges in a bleach solution of 1 gal bleach to 10 gals water is recommended for extreme algae conditions.

I hope you can use this method to destroy any algae outbreak that may occur. Your best bet is to keep this from ever happening. Make sure you keep your water chemistry in check and shock your pool once a week with 1 pound per 10,000 gallons of water.

For those with recurring algae blooms, add a Phosphate Remover chemical and follow up with a regular treatment of a good pool algaecide.

If you are persistent in taking care of your pool, you won’t have a problem with algae.


Happy Swimming :-)


About The Author

Matt Giovanisci is the creator of Swim University and has over 15 years of experience in the swimming pool and hot tub industry. He is also an award winning web designer and has been featured on Martha Stewart Radio as a pool & spa expert. For more information, how-to articles, and videos about pool and hot tub care, please visit SwimUniversity.com