Brown spots

I am new to this forum and new to caring for my pool. I have a 5000 gal fiberglass in-ground pool. I have been struggling with brown (mineral spots?) in my pool. I can't get the pool balanced for more than a couple days and then these brown spots start forming. They are heavy on the flat surfaces and also on the walls. It will not brush off. A local pool company sold me a bottle of ascorbic acid powder that was about $49.00. When I put it in the pool the spots immediately disappeared. They never told me how to prevent it from happening....only what would fix it. Cheap vitamin C fixes it immediately like magic. HOW DO I STOP IT FROM FORMING?