Algae growth

Algae problems in swimming pool water.
Green (cloudy) water or slimy pool walls.
Black algae. Mustard algae. Pink or white pool mold.

Algae growth

Postby dboys2565 » Sun 05 Jul, 2009 10:31

Looking for some help with an algae bloom in my pool. I had the water tested at a local pool store and the kid was unsure what to do to cure the problem. Below is the tested results of the water analysis.

[b]FC - 5.8
[b]TC - not tested
[b]ph - 7.6
[b]TA - 120
[b]CH - not tested
[b]CYA - 100
Phosphates - 1000ppb

[b]My pool 19,800 gals. with a vinyl liner. pool does NOT have a heater
[b]Pool chemicals I use 3in. Pace chlorine tablets in an automatic in-line chlorinator
[b]My pump & filter I have a Hayward 270 sand filter. Sand has been changed this spring with the new Zeosand.
[b]Other info - The pool water started to get cloudy and in 2 days I got an algae growth on the botton of the pool. The pool stor said to drain 6 inches of water and add Phos-Free and "It should, probably" clear up (not very convincing)

Any help or suggestions will be greatly appreciated.

chem geek
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Algae growth

Postby chem geek » Sun 05 Jul, 2009 14:29

Your continued use of HTH Pace 3" Stabilized Chlorine Tablets has increased the Cynauric Acid (CYA) level making the chlorine less effective. The 100 ppm reading from the pool store may actually be 100+ depending on the type of test being used (i.e. you may have more than 100 ppm CYA). The following are chemical rules of fact that you need to keep in mind when using chlorine products:

For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.
For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.
For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by 7 ppm.

A little simple math shows that even at a low 1 ppm FC daily chlorine usage, in 6 months the CYA from Trichlor will increase by over 100 ppm if there is no water dilution.

You will need to do a partial drain/refill to lower your CYA level. To get rid of the algae, you will need to shock with unstabilized chlorine, preferably chlorinating liquid or bleach (unless your CH is very low in which case you could use some Cal-Hypo for a while, but need to pre-dilute it in a bucket of water). Whenever you add concentrated chemicals to your pool, do so slowly over a return flow with the pump running and for extra safety lightly brush the side and bottom in the area where you add the chemical.

You should also get your own good test kit so you can take charge of your pool. Test results from many pool stores are notoriously inaccurate. Please get yourself either the Taylor K-2006 you can get at a good online price here or the TF100 from here with the latter kit having 36% more volume of reagents so is less expensive per test.

Read Defeating Algae in the Pool School to learn how much to shock your pool -- you need to raise the FC to a high sustained level. In your case, you may want to do a partial drain/refill before shocking to the high level since a lower (diluted) CYA level will require a lower FC level for shocking.

A phosphate remover will not remove the algae -- it will only remove a nutrient the algae needs to grow and it's expensive. I have 2000-3000 ppb phosphates in my pool and keep algae from growing by simply maintaining an appropriate FC relative to the CYA level. I don't need any algaecide either and only use 12.5% chlorinating liquid in my 16,000 gallon pool costing me $15 per month -- that's it except for a small amount of acid added every month or two. I have a pool cover so have relatively low 1 ppm FC per day chlorine usage. However, if you don't think you can maintain your chlorine level in your pool, then use of a supplemental algaecide or phosphate remover or 50 ppm Borates can be like insurance against algae -- not necessary, but an option.


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