The living quarters for your other pets must be cleaned on a regular basis. However, the water in a standing fish tank need not be changed. The waste products of the fish make the water better for them to live in. People who have aquariums will tell you about two kinds of dirt in a tank. There is clean dirt and dirty dirt.
Clean dirt is the waste products of the fish.
This does not need to be removed. Dirty dirt means the uneaten food and the bodies of fish that may have died. These kinds of things need to be removed. Beginners will have to start with plain tap water. Letting the water stand until it comes to room temperature will get rid of any chlorine that may have been added to the water to purify it.