I dont know the Nikon line too well, but the canon T3i can be found for some good prices. The biggest recommendation I can make for fish tank photography is a tt560 from Neewer, and a remote flash trigger from cowboy studios. If youre looking for general photography accessory ideas, the 50mm f1.8 is an amazing lens for cheap. Also pick up a reflector or two.
Canon Pimp Club #072
The Fraternity of Dirt #87