If you haven't already, you should buy a gravel vac. It's just a siphon for sucking up debris on the tank bottom. They don't cost much ($10), and they really make water changes easy on you and the fish. My fish have never been bothered by the vac at all, they'll swim right up to it and all around it. They're quite interested to see what I'm doing. Keep at eye on the fish if they're small though, as they could be sucked up too. My platies are now a little too comfortable with the gravel vac, and are trying to explore the inside! Water changes are much more effective if you remove the accumulated debris in the substrate substrate with a vac.