There are a couple "tricks" that tend to be fairly effective. You can either use some lacquer or some strong rubbing isotropic alcohol. Just rub on and allow to sit for a few hours.
Drain pool of water first. Scrub strains with sponge. Products you can use to spray is lacquer, metal remover cleaner and enzyme chemical pool cleaner to get rid of stains.
First try scrubbing with a stiff brush. If that isn't enough, buy an enzyme chemical pool cleaner or an ascorbic acid cleaner. To prevent in the future, have your water tested for metals.
