sandraca

We are using a Healthcare Pool System and this year have been having streaks of dark stains on the fiberglass surfaces. When we treat with PH- it takes out the stains but they return again. I'm wondering if this is from the ions in the water from the Healthcare system. We have been chlorinating with bleach but our water is very high in ph so we have trouble keeping any measurable chlorine levels. I hope someone is able to shed some light on this problem. We don't have any problems with algae though! Thanks, Sandra