Nearly everything releases phophates. It depends on how stocked your tank is. What size and fish, inverts etc. do you have in your tank? I have a 55 that I am switching to a 150 soon but only have a small angel, 2 clowns and a black cap basslet in my tank. I have had to use tap water numerous times also. Phosphates are just a part of having a tank. You can do cheap options like pads or better stuff like granular products that you place in your filter. This should get rid of them till you start using RO water if you already are not. Also the planting of shaving brush and mangroves seems to have helped mine. My nitrates, nitrites, ammonia, phosphates etc are all undetectable by every test imaginable.