I guess I don't see it as "constant". If the two fish in there double their size in a year (which obviously they haven't done in the last 5) I guess I would only be looking at one move and anything can change in a year. As far as the undergravel filter - it has been in there 11 years with out moving. I don't plan on using it, but from my reading other people recommend their use with Angel fish because they don't create a current. Canister filters do sound interesting. The available one is for a 75g tank, but I can't use it anyway because there is no way to get it lower than the tank.