I don't have fish any more, but when we did their tank got cleaned out about once a week. Basically whenever it looked in need or doing and/or started to smell a bit off.
i change mine every 1-2 weeks, usually 1 1/2... i dont know if i should be donig it more often or not, but it seems to be living healthily
When you change the water only change 2/3 of it.. Keep some of the old water in there so it isnt a shock to the fish
