Changing your fishtank's water is extremely important. The thought of how many people go day to day without changing their tank water is frightening. Water changes should be done once or twice a week depending on size of the tank and amount of fish or what kind of fish. Goldfish produce twice as much waste as tropical fish so they definitely need to be in a separate tank or you'll need to do a water change twice a week to keep up with the nitrate levels. A water change doesn't just get rid of waste, it brings new oxygen into the tank which is an extreme necesity for a fishes survival. People think having a filter and a bubbler will get rid of the need for a water change. And if that is what you think, give your fishtank to someone who actually cares enough to take the time to learn how to properly take care of fish. I cannot stress this enough. A fishtank is enclosed ecosystem so the fish count on you, as their owner, to dispose of their waste like you would with any other pet. Even in oceans the water is replenished by the water cycle and by the earth's crust where valconic rock will boil the water and act as a filter. A simple water change won't kill you. Take out 10-15% and make sure you condition the new water to get rid of harsh chemicals like chlorine found in tap water. Please take fish keeping seriously- fish matter too.