Fish tank water change, Freshwater aquarium maintenance guide. A detailed guide on how to do a water change on a fish tank or freshwater aquarium. Doing regular water change on any fish tank or aquarium is very important to maintain a healthy living environment for your fish. Regular water changes once every week or every fortnight helps maintain harmful elements like ammonia, nitrate and nitrites in check which on excess can be harmful to your fishes. Fish tank water maintenance can also help maintain a healthy level of beneficial bacteria which can prolong the life of your fish.