Tapwater

[DCDN]

Once I switched over to ro water and not the bottled type I saw a major difference in the amount of algea in the tank. I wouldn't go back now. Though I have heard people very happy with just tap water.

[MMAX]

It all depends where your tap water comes from. Test the phosphate and nitrate levels before you use it. I tried it where I live 2 summers ago with disasterous results. Didn't kill any livestock but had a major cyano outbreak. Had to do a complete tear down, scrub and put back together. Good thing it was only a 20 gallon. I use nothing but RO/DI.