if I can see it from the outside of the tank I clean it, but I leave the inside alone unless it gunks up the flow, or I can see a bunch of algae. If there is a bunch of stuff gunking up, I take a towel and just wipe it out, I don't even use warm water. About once a month or so, I do take off the intake tube and swipe with a brush (you can buy these at lfs), to make sure flow is good.