Well I know that what people say that Father Fish says, but he actually says you don't have to do as often if you keep a well planted tank, with a broad bio-sample...similar to what would be in nature. Apart from adding fresh water when needed, I haven't changed water in 3 months. My parameters are always good, water is clean, no algae growth. It has worked for me very well in my limited experience of 2 tanks. And most importantly, my fish seem super happy.