PerfectReign

Last March, we had our pool redone. We went from plaster to fiberglass for the surface. We also switched from pure chlorine to a salt system. All has been going well, until earlier this summer. About late may/early june the pool guy (me) noticed that there were darkish stains on the spa walls. I brushed (which I do every few days) and then added a little bit of liquid chloine to see if the stains would go away. So far they haven't. I only see them on the spa so far -no like stains in the pool. We generally don't heat the pool and it stays in the low 80's from May to October. The spa gets heated on occasion, but we have the thermostat set to 90, since I have rugrats who use it. Any ideas on what this is? I've googled and searched the forum but not found anything. Again, the pool is an in-ground built around 1990. We converted it to fiberglass last year from plaster.