Thanks. I use either a 60 mm f/2.8 macro on larger fish or a 105mm f/2.8 micro lens for smaller fish, mostly. I have a number of different lenses, but the 2 I listed are my favorites. I use a 28mm f/1.8 for tank shots.
Overhead flash is the secret, I kid you not. Nikon has commander mode, a feature that can trigger wireless speedflashes. I believe with Canon, you need to use an infrared transmitter that costs a few hundred bucks to fire remote flashes. With either system, you can wire your flash to your camera via a cord and still place it on top of the tank, but you're limited to the length of the cord you use.
Most of any money invested in anything other than entry level dslr will go predominantly into lenses. If you become serious about it, kit lenses won't cut it for you. It's a rather addicting hobby, lol.