Green algae on walls

[MesaAZUSApoolowner]

I have an “interesting” problem. I own a 25,000 gallon in-ground pool in Mesa, AZ (USA) and last week I noticed that the walls and floor were turning green with what I assume was algae. However, the water remained relatively clear and the chemical levels were fine: chlorine level was 4 ppm throughout week, ph was 7.6-7.8, alk was 90 ppm and cyanuric acid is 50 ppm.

This weekend, I vacuumed the floor and walls removing all of the algae. Unfortunately, there was some filter bypass and the water became a little cloudy. After vacuuming, I adjusted the ph to 7.5 (with muriatic acid), added some clarifier and left the pump running for 24 hours. Yesterday, my pool was crystal clear again.

I’m stumped about why algae developed in the presence of high chlorine and acceptable alk and ph readings. I’ve been very diligent this year to not let my chlorine level drop in order to prevent algae. Does anyone have any theories on why this happened and how I can prevent it in the future? I ran a search of the archives and didn’t locate a similar post.

Some additional info on my pool:

Filter: Hayward 175 sf cartridge one year old
Pump: 1 ½ hp AO Smith
Pump run time: 8 hours in summer but just increased to 10.
Water temp: 86 degrees F
Pool surface: Painted. Chlorinated rubber one year old. No noticeable chalking or peeling.

Thank you.

[Buggsw]

Hey, fellow Arizonan!

What are you using to test with? If strips, use a real test kit or take a sample to the pool store and post your readings again.

You may have a chlorine reading of 4 but if you have a reading of 4 in combined chloramines you have a net of 0 Free chlorine and algae can grow.

Also, check for phosphates in your water.

What is your CYA? If it's high, it reduces the effectiveess of chlorine and you need to keep even higher levels for sanitation. If your CYA is high, you should not use tablets nor pool store shock. You should switch to liquid chlorine.

If you post all your readings (including Total Chlorine, Combined Chlorine and Free Chlorine as well as CYA, pH, TA and phosphate level) I can tell you how to treat.

[Michael Silvester]

Hi Mate,

All you have to do is give the pool
a shock dose of chlorine.

While the shock dose is in the water
you need to brush the walls and floor
everywhere you can making sure you
dont miss any spots.

Then add a copper based algaecide
to prevent it happening again.

Make sure your pump is running
while your doing this.

Algae that is stuck on walls and the
floor have only 1 side exposed to chemicals
thats why you need to scrub them off
and get them into suspension so that
the chemicals can attack from all sides.

Hope this helps?

Take Care,

Michael Silvester

[MesaAZUSApoolowner]

Thank you for the responses.

I am using a drop test kit (HTH) that measures total chlorine, ph, alkalinity, cya and calcium hardness. I do not have a kit that measures FC and CC so I’ve relied on the “eye” method i.e. the reading within the first few seconds of oto addition is free chlorine and after a few moments it’s total chlorine; the difference being CC. Also the “smell” test: If there’s a chlorine odor, it indicates the presence of chloramines. Not the best method, I know! My total chlorine has been around 4 ppm with not much discernible difference in the two readings. Needless to say, I need to acquire a better test kit.

The phosphate issue is something I’ve thought about however, there appears to be a lot of debate in the pool forums on how to treat and if it’s really even an issue for the majority of pool owners. Of course I never take the advice of “pool store” people; if I/we did, we’d go broke and still have ugly pools! That being said, I will take your advice, go down to my local pool store and have them run a phosphate test. I’ll post the result here and not listen to their advice/sales pitch.

My cya is 50 ppm. I do not use the “pucks” because of the “over stabilization” effect. I use liquid chlorine exclusively. Usually the 10% pool formula or, if there’s a sale, the 6% straight laundry bleach. I am adding ½ gallon of the 10% solution every evening to add 2 ppm to my 25,000 gallon pool.

I vacuum my pool once a week running the vacuum up the walls to the tile line and across the floor. I only brush the areas that the vacuum can’t reach well. The vacuum does a good job of pulling the dirt/algae off the walls. In the past when I’ve brushed, it created a big cloud of debris in the water that I like to avoid. When I had the problem last week, some of the vacuumed algae slipped through the filter suspending itself in the water making it a little murky. However, I could still see the bottom drain. After vacuuming, I adjusted the ph, added some clarifier to coagulate the algae particles and then chlorinated at sundown. The next day my pool was clear again.

Today, my pool water is crystal clear and the walls / floor are no where near as bad as they were last week. But I can still see a few green dot areas.

I’m inferring from both your responses that I probably have a combined chlorine issue that I first need to test for and then shock to eliminate. I will purchase the appropriate test kit, shock and keep you posted.

Thank you for taking the time to respond!

[Buggsw]

You are going to have to shock it and shock it well day and night.
With your level of CYA your shock level should be at least 15ppm.

For your pool volume of 25000, you will need 4.6 gallons of regular strength Chlorox 6% more if you use an off brand of bleach, possibly. Or, 2.8 gallons of 10% pool store chlorine.

In the morning, if the reading has stayed at 15ppm you can let the chlorine fall back down. If it has dropped, you need to bring it back up to 15 or more throughout the day and the next night.

It takes 2.1 gallons of 6% to bring your pool up by 5ppm.
It takes 1.2 gallons of 10% to bring your pool up by 5ppm.

Keep your pump running and your pH at around 7.2, your chlorine to at least 15 until it holds overnight. Once it holds, you should be good to go.