The frequency of cleaning your molly fish tank depends on various factors, including the tank size, filtration system, and the number of fish. In general, a partial water change of 20-25% should be performed every two to four weeks. Regularly remove debris, excess food, and maintain the filtration system to ensure water quality remains suitable for your mollies. Additionally, monitor water parameters such as ammonia, nitrite, and nitrate levels to determine when cleaning and water changes are necessary. Adjust your cleaning schedule accordingly to keep the tank environment healthy for your fish.