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Every now and then, depending on the "lighting" I see what looks like dirt stains on the bottom of my plaster pool. At night with the pool light on, the bottom looks beautiful, but other times (cloudy outside) it seems like I'm seeing stains at the bottom. Bright sunny days "hide" this phenomenon as well.
Is this just an optical illusion from the cloudy skies? It's wierd, I know, but it looks like brown stains. Maybe I need a vacation?
No, your eyes are not playing tricks on you - it probably is a mineral stain. You can take some vitamin C tablets and crush them up in a sock, or ph down, and rub it on the stain - if it takes it up it is a mineral stain. You can take your ph down to 7 - 7.2 and put in some metal remover (sequestering agent) and it should lift the stain. Let us know if this takes care of the problem. I have a fiberglass pool, and if it has stains you mostly see them in the pool when there is no sun on them.
What makes testing the stain difficult, is if the lighting changes, the stain disappears. I'll give it a shot though.
If it is that hard to spot, then I would ignore it;)