First off, yes, you still have to change the water. I use this stuff on my Fiance's tank because I cant always get over there for wc's. It has some kind of nitrate absorbing/exchange resin crystals in it that SLOWS the build-up of nitrates. I put in a dose before I leave for a couple days, then when I get back, the water is exactly as I left it. I would not dose the tank every week & leave it for 6 months like the package says. It may buy you a week or two...max. It has some other good stuff in it though. Trace minerals, pH & KH stabilizers, blah blah. 1 big bottle has lasted me easily over a year. BTW, it turns the water yellow for a few hours & you can see the crystals floating around until they settle. Great for checking flow pattern in the tank!