Low calcium ~ 225 ppm?


Well-known member
Apr 3, 2016
Baltimore MD
I was wearing goggles last night and I noticed that my plaster has some brown spots. When I rub my finger over these brownish yellow spots I can tell that there is worn away plaster as there is a minor indent. Now to clarify, my water is crystal clear and the bottom plaster looks incredible when you are standing next to the pool. I say that so you know this is rather minor. Only with under water inspection will you notice this. Otherwise it just looks like a little dirt, that I now know won't go away with vacuuming.

I just tested calcium at 225 which I thought was still in the OK range but it might be a bit too low. I'm going to add in quite a bit of calcium today but I wanted to know does it sound like low calcium might be causing this plaster issue? Some pulling off plaster off or etching b/c of low water calcium? Or is this just pool aging. It's 5 years old.
The most recent workup was:
CA 225
TA 60
pH 7.8
CYA 25ish (lower than 30)
Chl - 2
temp - 87

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
Your test numbers provide a CSI level of about -0.04 which is outstanding. Not nearly negative enough to be a problem, so if that's the norm for you, I don't think water chemistry has anything to do with it. Five years is a bit young for plaster issues, but you never know if there was a spot or two that may have had some inconsistency during the mixing or application process. But I have to ask ... these indentations you notice, is there any chance they have always been there, but you only notice them due to a brown/yellow build-up material that settles within them?


Well-known member
Apr 3, 2016
Baltimore MD
You may be right. A lot of them have the swiping motion of someone smoothing plaster. Here’s 2 pics. These pics are pretty up close, so the staining in the pic is probably 6-10 inches from left to right in the view.


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Last edited:


Bronze Supporter
Mar 5, 2020
Melbourne, Australia
Pool Size
Salt Water Generator
SWG Type
Astral VX 7T
As Texas Splash said, at the moment your CSI is pretty perfect, and with pH above 7.5, it will stay in the ideal range. What he explained, is probably what's going on.

But your CH is quite low, so a few things to consider:

Your CYA is quite low right now, you probably have it higher usually. Adding CYA will replace carbonate alkalinity with CYA alkalinity (TA will stay constant), this will decrease CSI a little. Not a big effect, but to be considered in the chain of effects.

Water temperature has a big influence, more Ca can get dissolved in cold water, i.e. lower CSI. Consider winter temperatures, that's where your pool spends a lot of time. Water temperature of 40F puts your CSI down to -0.47. Increase CYA to 60, and your CSI will be -0.59, decrease pH to 7.6, and CSI will plummet down to -0.78.

Have a think about the water parameters your pool spends most of the time at, and calculate the CSI for a few scenarios.

I have very low CH and TA in my fill water. Over winter, I maintain my CH higher than in summer to keep CSI up. Over the course of winter, I have to keep adding Calcium Chloride to compensate the losses by rain overflow. My pool remains open all year.
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