Big Algae and Peeling Cement Problems


New member
Apr 6, 2012
Early 2011 I realized that my in-ground pool had a low pH, down in the four range. It was installed around the late 1980's. I started adding soda ash to bring it up. It has taken since then over 60 lbs and still is only in the mid 6's, falling again after additions. Since then I have had purple coloration, the forum suggests this to be manganese, which clears with time. But two things bug me badly. First is that for a year I have battled algae and to the point of failingly in the summer. Never a problem to me in the past. Chlorine is the base plus some shocks and quaternary ammonium algecides. And now the cement lining is “dissolving”. I did replace recently about one third of the water, but no noticeable improvement.

Were my two problems due to an attempt at pH adjustments?

And is my only recourse to drain the pool and have the cement replaced by a pool company?

Any comments appreciated.


LifeTime Supporter
Platinum Supporter
TFP Expert
May 7, 2007
Silver Spring, MD
Welcome to TFP!

The damage to your plaster is from the low PH dissolving the plaster. Chlorine is also far less effective at very low PH, so that may be how the algae got started.

The first priority is to get your PH back to reasonable levels.

It would help if you post a full set of water test results. Can you also tell us what specific chemicals you have been using? I suspect you may have been using dichlor, which is very acidic.


New member
Apr 6, 2012
The plaster damage only started once I started bringing the pH up. Maybe it's a coincidence as the plaster is 20 years old and not really pH related.

Most of the chlorine is indeed sodium dichlor S triazinetrione dihydrate (presume this is what you mean by dichlor?). The guy in Leslie's told me it is acid as is the liquid sodium form but that the calcium hypochlorite is not. True ?

I have been using sodium carbonate to adjust pH.

Some further analysis:

pH 6.8 or 7.2 depending on who did it.
Free chlorine 0.5
Total available chlorine 0.5 -- added 2 lbs potassium monopersulfate
calcium hardness 360 ppm
Cyanuirc acid 99 ppm
total alkalinity 140 ppm
TDS 1900 ppm
Phosphates 0.

So the he say to add hydrochloric acid to bring down the hardness then soda ash to re-adjust the pH. Does that make sense?



Mod Squad
LifeTime Supporter
Platinum Supporter
TFP Expert
In The Industry
Apr 1, 2007
Sebring, Florida
It puts you in the middle when you ask us to second guess the Pool store advice. You will do better getting your pool clear to either follow their advice our follow this forum but don't try to blend both for several reasons.

Adding hydrochloric acid to reduce hardness wouldn't make sense to me... Your pH is already to low and you have some experience with the problems that causes