I have a brownish stain around the entire top of a white fiberglass spa. It's about a half inch in height and goes all around the spa at the water level. They have no noticeable thickness and are translucent. They cannot be felt at all if you run your fingers over them.
The spa is bromine based. Current tests show:
Bromine: 5.4 ppm
TA: 140 ppm
I started the spa with brom start (NaBr) and every day have been adding bleach to bring bromine levels up to about 6 (then they decline overnight and during the next day). After adding the bleach, I test pH and add muriatic acid to balance to 7.4-7.6.
What is the likely cause of the brown stains? My guess is some kind of metal, but I'm not sure. I have a bottle of spaguard stain and scale control... would this be useful? It doesn't say what it is... is it a metal sequestrant?