depends on the tank. the 1 gallon, i take the fish out and clean it and let her reacclimate from her little cup back into the water. the 2.5 gallon- depends on how dirty i think it is. sometimes i just do a 50% water change, other times i take her out and clean the whole thing. the 5 gallon, i just do a 50% water change on. too much of a pain to disconnect everything.