Alniter

I have a Blue Hawaiian fiberglass pool with a saltwater system that I had installed new 5 years ago. We've always loved it. The biggest problem we've ever had with it is metal stains, which we've treated successfully twice with ascorbic acid, the last time of which was two months ago. Since we have a screened pool deck, we don't get much dirt in the pool - just small particles that blow through the screen during windy days or storms. However, what used to just brush off the pool floor is suddenly sticking to it! It's very hard to brush off, using your feet gets a better result, but it seems like nothing will get it all off the pool floor (and the sides at water level). Could this be a gelcoat problem? The surface of ours is guaranteed for 20 years, but could the ascorbic acid treatment have damaged the surface this time? Our water tests are always fine and we keep up with maintenance (algaecide, metal treatment, et cetera). Any clue as to what might be causing this? Thanks, Steve in Florida