Black Spots .... what are they?

[almadillo]

I have 4 black spots on the plaster of the pool. The pool was re-plastered last year (Sept) then closed for the winter.
The first spot appeared a few weeks after opening this past spring. Then the other 3 showed up a few weeks after.
2 are on the bottom. 2 are on walls.
If you look at them closely (zoom in), they are quite hard to see and semi-transparent. They are much more visible from farther away.
All are the same size .... about the size and shape of a dime.

Any ideas what they are and how to get rid of them (and eliminate future ones)?

 

[Texas Splash]

Welcome to TFP! Spots and stains can be from any number if items or products. Generally the first place to start is a with a full set of reliable water test results from your TF-100 or Taylor K-2006C test kit. If you didn't see those type or marks before the re-plaster, hopefully that will rule-our any type of rebar staining from below. For now though, consider the following:
- Rub a chlorine tab on it (organic/algae)
- Rub a Vitamin C tab(s) on it (iron)
- Rub dry acid on it (copper)

Tell us how you typically chlorinate your water, if you use pool store products, or are on a well. And don't forget to update your signature with all of your pool and equipment info.

 

[almadillo]

Hello. And thank you for the reply.

After the re-plaster last year, we bought water from a pool-filling service as I am on a well. I do top off the pool with well water as it gets low.

Water levels as follows:
FC = 5.5
pH = 7.8
TA = 70
CH = 250
CYA = 60
Salt = 2900

I have a salt generator running.
Only use liquid chemicals (bleach, muriatic acid, etc.). I add a scale & stain remover weekly.

I tried a vitamin c tablet but it did not dissolve well (coated I think) and after sitting on the spot nothing changed.
I'll try a chlorine tab. Thanks for the suggestion.

 

[Texas Splash]

You might still try some generic Vitamin C tabs without the coating just to make sure seeing as you are on a well. You can try to apply directly, or crush enough of them in a thin sock or nylon which may help.

 

[onBalance]

Those dark spots may not be a "stain" - but rather a discoloration of the plaster itself.
If, when plastering, a small leaf or something organic blows into the pool and gets troweled into the plaster coating, then over time, the leaf will decay and bleed through to the surface, producing a small and round dark spot discoloration. That is one possibility and what may have happened. If so, then the spot treatments with vitamin C may not remove it. It will be interesting to see if the chlorine tabs "bleach" the possible organic discoloration and make it disappear.