help identifying a stain

Hello, all! I'm a second year pool owner (20 yr old 20'x40' gunite) that got reasonably up to speed last year by poring over this amazing forum. I can't express how grateful I am for such a wonderful resource!

Sorry for the long-winded description below. Since I'm inexperienced enough to not know which details are relevant and which aren't, I'm erring on the side of too much information.

I'm in the process of opening my pool, and have a significant amount of blackish staining at the bottow of the deep end that I didn't encounter last year. It doesn't vacuum up, and hasn't cleared up any with sustained appropriate FC, PH, and alkalinity levels in the water over the last week. I've ordered a metal stain ID kit to find out if it's due to iron, and I believe if it's just dirty organic-based filth the chlorine will eventually take care of it, right? Are there other possible causes that I don't know about?

When I first took the cover off the pool, the water was relatively clear, but there was a fair amount of yellow-brown algae/sludge coating the pool surfaces (mesh cover - I'm not a fan). However, at the deep end was a dark greenish blue haze 8"-12" deep that obscured the bottom surface of the pool. The algae coating the rest of the plaster did not extend into this haze - after clearing up the water there was a perfect bowl of non-algae at the bottom where the haze had been. This is also where the stains are. They kind of look like powder burns, in some places random/splotchy and in others running in distinct ooze-like patterns. So my guess was that the haze was algaecide that the pool guy that helped me close the pool added, which settled down there. But can algaecide cause stains like this, or is the co-location just a coincidence?

Hi mappo, and welcome!!

I can't really answer any of your questions--just wanted to mod your post in so that others who may have answers for you can see it, and welcome you to the group. If your stain is organic in nature, then eventually high chlorine levels will lighten it--whether or not it will totally remove it depends on what's causing it. You have a gunite pool, so at least you can rule out mold or fungus growing under a liner and seeping through, which would be really bad. It sounds to me like the algaecide is probably the culprit, but again, I'll leave the diagnosis to those who know better than me. Do you by any chance know what kind of algaecide he added?

Thanks for the reply and the welcome, aylad. No, I don't know any specifics about the algaecide used.

Black stains that exclude algae sound like copper oxide -- copper algaecides, copper pills, ionizers, Nature2, etc *OR* a pool heater PLUS low pool pH levels can all lead to that problem.

Put a vitamin C tablet on a lightly stained area: if the vitamin C clears it somewhat it's metal in origin and probably copper. Black iron stains are ferrous and soluble, so they are not common in pools.