I use my tap water (well water), but ive been using it for as long as ive had my tank and i havnt had any other problems with algea except on this coral. I dont think its phosphates because it would be growing in patches that think everywhere in my tank and only spot i am seeing that type of algea growth is on that coral, even look on the rock i put the coral on in the pic (its been on that rock almost since ive had it)