I use my tap water for my salt and fresh tanks. I never had a problem with my salt tank thus far using tap water. I keep my water in a tub, mix in salt, keep an airstone in it... and let it sit for 24 hours before I use it. My one coral, one fish, and multiple inverts are fine. My phosphate test shows 0 on the tank and 0 on the tap. I have city water. I dont use an ro system... as I has no monies for one. I got most of my stuff when I actually had money. I use prime on my tanks.