I have a Nikon D5100. The kit lens is an outstanding lens, you're actually very hard pressed to find any lens in that focal length that will significantly outperform it. A bigger aperture would be nice, but its not really worth the extra thousand it would cost you if you're not really serious about photography.
For aquatic photography, I set my camera to M. I use 1/200th shutter, ISO 320, auto white balance and f/8-f/11 depending on where the fish is in the water column; lower f/stop for the deeper the fish is in the tank.
The fiirst key to getting GOOD photos is remotely firing your flash from above the tank.
The second key is PRACTICE.
Good luck!