I'm still working on picture quality too - so I know how you feel. I do have a suggestion though - instead of a tripod try pulling up a chair to the tank and rest your elbows on your knees or on the tank stand if there's enough of a ledge.
My camera doesn't have an adjustable zoom, there's just the normal setting, and 2x zoom. The automatic focus doesn't work too well either.
Flyingfish - actually, the picture above was taken using a chair as a "tripod". It heps a little bit. My hands aren't very steady, so I definitely need some kind of support.