First let me say how happy I am to finally be able to log in to a pool forum, I've been interested for a long while and kept checking the "Pool Solutions Forum" site. I first stumbled on to it about 5 minutes after it was locked from any new members.
I have a 14' x 28' inground vinyl lined pool.
My pool is about 4 years old. Sometime in the 2nd year I started seeing a dark stain appearing around the main drain. The stain goes about 1/3 the way around the drain and about 2 inches out from the drain, on the deep end side of the drain. I noticed it seemed to fade some when I shocked and returned after as the higher chlorine level subsided. In the middle of the season in the third year the stain started to get bigger, about 1/2 way around the drain and 6 inches out.
I called the builder to come have look, they thought it was black mold. They had me drain the pool, then they came out and put on a new main drain gasket, mixed up some bleach and muratic acid directly on the stain, let it sizzle for 10 or 15 seconds and then started filling the pool from the water truck.
This cleaned most of the stain, there was still a tinge of brown, but a lot better. Now, a year later the stain is back to where it was at year two (1/3 the way around and 2" out), doesn't seem to be getting any bigger than that so far...
Does anyone disagree with the black mold diagnosis? Any thoughts as to why it's only on one side of the drain?
This detracts from the clean pool I try maintain.