Whay does a fish tank produce phosphate?

Well, it's not the fish tank producing phosphate but what you put in it. Some of the things that will produce phosphate are flake and frozen fish foods, bacteria, decaying plant matter, fish feces, fish slime, carbon, pH stabilizers and a few other things. Doing frequent water changes and keeping feeding to a minimum will help reduce your phosphate. If you have lots of algae, you may have elevated levels of both nitrate and phosphate. Goodluck!