shock it, usually the chlorine in low after lots of swimmers or a big rain, it either is low in ph or chlorine. Either way shock it helps. Go to your pool supply store and get an algecide that contains about 20% copper solution and 80% algecide. Add the required amount and chlorine shock the pool. It may take a day or two but it works for me every time. Ernie This usually involves checking the Ph levels or more likely algae. Chlorine levels may help but check with a pool cleaning company. Maybe muriatic acid would help, but it usually has to do with algae and proper flow and filtering.