How often to clean a 5 gallon tank?

If you look after the tank properly it should only need a major "cleaning out" once a year or so. To do this, you need to follow the basic rules of fishkeeping which are. :- 1 inch of fish needs at least 1 gallon of water. Every tank must have a permanently running 'cycled' filter. :- Every tank must have at least 50% of its water replaced every week.