I have 150 G tank with a fuge and a skimmer rated for a 220G. I would say around 100lbs live rock and about a 2 inch bed of live sand on the bottom of the tank along with my fuge filled with sand, rock, and chato. I had my tank for over a year and did not do a single water change besides topping off with fresh ro/di water. I HAD about 4 different corals and mulitple fish. I would add I say a fish every month or 2 and had no problems with my water quaility. I would take samples of my water in to the local fish store every month if not twice a month and the water quality was spot on. So one day I get the bright idea to do a water change (this is about month 14) then all my corals slowly started to die and then the fish started disapearing. So here is my question. If you have a natural system ie a REEF system are water changes needed?