Battling Green Algae

by Katri Tuomala
((Grimsby, Ontario, Canada))

Green algae took over our pool a few weeks ago. I've been trying to clear it up before closing the pool for winter.

Yesterday I added approx. 10 lbs calcium hypochlorite granules in the pool pre-mixed in pool water. This morning the pool is still green and the test strip reading are:

free chlorine: between 5 and 10 ppm

ph: 8.4 or over

total alkalinity: between 80 and 120 ppm

cyanuric acid: low

We have sand filter and the test strip does not have water hardness value.

What would you recommend next? Or should I just drain the pool to winter levels (above ground pool), cover it and start over again in spring?

Please help!


Thanks for the question Katri

I would advise you to go ahead and close the pool for the winter. Clearing it up would take a week or two. That's alot of wasted chemicals and time. Just be aware that you'll probably have a mess in the spring.

When you do get an algae bloom and want to shock the pool, it would be best to use liquid chlorine instead of chlorine granules. They have a pH level of 12. This is probably why the pH of your pool is so high right now.

Clearing up a green pool is a process, not an event. Sometimes it takes more time and effort than the next pool owner who has a similar green pool. You need to keep the chlorine level at 12ppm, or above, for a period of time.

Even though you shocked the pool and the chlorine tested high, it didn't get all of the algae. The algae that remained can reproduce quickly. Keeping on top of the chlorine is the best defense against another outbreak of algae.

Here are some links that you can look over:

Green Pools

I Can't Get The Green Out Of My Pool

My Pool Water Is Green

There are many more answered questions about green pool water on the Q&A page:

Swimming Pool Questions and Answers

Be sure to get a good test kit. Strips are fine, but for the most accurate readings, you want to get a Taylor FAS-DPD K-2006 kit:

Pool Water Testing

Water Testing Kit

It's best to get the pH down to 7.0 - 7.2ppm before you shock. The chlorine is much more active at these levels.

If you have any other questions please feel free to contact me again.

Best of luck with your pool.


Comment By Katri

Thank you so much for the advice. I will close the pool for the winter. Could you tell me what to expect in the spring. Do I fill the pool and start shocking with liquid chlorine until the algae dies? Whats your estimate, how long it will take? A few days, weeks? At what point do I start backwashing?

After all the algae is killed? This pool is 30 x 15 x 4 feet. What's the amount of liquid chlorine to be added initially? And then do I keep making sure the free chlorine is above 10 ppm until the algae is killed? I really appreciate your help.

We've had the pool for 5 years and have not had a big problem like this with it until now.

Comment By Robert

There's a good rule that pool guys use, it's close late and open early. Many times people close their
pools in early to mid September when the weather can still get warm. A closed pool with a cover and warm weather is the perfect breeding ground for algae. Close the pool as late as you can and open early.

In the Spring you can expect to have algae. When opening the pool, hook everything back up again and be sure there's no debris on the bottom. The skimmer and pump basket should be clean. Fill the pool to the proper level.

Get the pH down to 7.0 - 7.2ppm for reasons below. Your cyanuric acid (stabilzer/CYA) should be between 30 - 50ppm. The original question said it was low. I'm not sure the reading you got, but I will assume it was below 30ppm.

Shock with Dichlor. This is a stabilized form of chlorine. Retest the CYA after 8 - 10 hours of filtration. Once it gets to 30 - 50ppm, switch to regular liquid chlorine. If the CYA is already in line, just use liquid. This is the link that you need:

Green Pools

It gives you a step by step guide on how to clear up the pool, but I'll give you the main points.

Shock the pool and brush it in the evening, then retest in the morning. You'll probably see the chlorine level come down. This means the chlorine is consuming the algae.

Keep a log and write the chlorine numbers down. Shock again in the morning, brush it completely, then retest in the evening. Keep shocking, brushing, and retesting until the chlorine only drops between 1 - 2ppm. At this point the algae is dead. Keep filtering and backwash once per day.

You have to stay on top of the chlorine (shocking) because, if you don't, the algae will come back.

It depends on how well you keep the chlorine level above 12ppm, but by doing the above, you should start seeing results in 2 - 3 days. The pool will go gray, then white in color.

You probably have an 18,000 - 21,000 gallon pool. You'll need 4 gallons of liquid chlorine to reach 12ppm. If the level goes higher, don't worry. It's better to go a bit higher than not hit the mark. Mix the chlorine in a bucket with water and broadcast around the perimeter of the pool with the pump on. Don't add chlorine right to the pool. It will sink to the bottom and stain the liner.

Keep filtering 24/7.

I've cleared up pools that only took about 3 - 4 days, others have taken between 7 - 10 days. It really depends on the pool.

Here are some other links for you:

Green Pool Water

Swimming Pool Algae


Swimming Pool Chlorine

Pool Shock

Pool Chlorine

Chlorine Tablets

Pool Chlorine Tablets

This is why I said clearing up a green pool and shocking is a process, not an event. And this is where many pool owners fail and frustration sets in. They do it once and expect the pool to clear up. Then they go to the pool supply store and buy $300 worth of chemicals. And the cycle begins. Green pool = more money = green pool, etc...

Sweep, keep the alkalinity between 80 - 120, pH to 7.0, consistent chlorine at 12 or above, filter 24/7, and backwash once per day. No algae can handle that.

Best of luck with your pool.


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