Frank, I would strongly encourage you to buy exactly what you need and avoid packages that include a lens you will not get the best out of.
As Mike and Pat already suggested, get a body, a macro lens, and a speedflash. The absolute necessities for taking good aquarium shots are a macro lens and a speedflash.
Most of the DSLR bodies are good for the beginner, but if you get into the hobby more seriously and start adding more lenses to your bag for the numerous situations that demand a variety of focal lengths, aperture speed, wide angles, etc., make sure you get one that can handle all the new and old lenses out there.
I would recommend Nikon or Canon brands. Both are strong competitors that are always trying to beat the other for the best. However, most folks who are into aquarium photography seem to prefer the Nikon platform because of the flash convenience that is made available with Commander Mode in the camera's menu and the built in triggering device to fire remote flashes. The Nikon Creative Lighting System is the envy of all the other competitors. And nothing works better for fish pix than a controlled remote flash shooting down from the top of a tank.
Body:
Go with either a Nikon D70, D80, D200, or D300 body. They are all priced upwards from the D70. And they all are compatible with all the lenses made by Nikon. Including the older lenses as well as the new G lenses. Getting other bodies may offer restrictions on which lenses you use. And, most importantly imo, all of these bodies have built in pop-up flashes that will trigger your Nikon Speedlights without wires or adding any other device when you use the flash off the camera.
Lens:
There are two main considerations when buying a macro lens. Size of the subject, and working area. The longer the focal length of the macro lens, the more space you will need between the camera and the subject. That's why most of us carry at least two macro lenses when we take photos of aquarium fish.
The two most popular focal lengths are 60mm and 105mm. Nikon offers both. They are both exceptionally sharp, and they are both f/2.8 aperture maximum lenses. Which means they are faster and provide better exposure then most of the zoom lenses that are normally bundled with camera lens packages. So, for your first lens, I would suggest the Nikkor 60mm f/2.8 D Micro or the newer G version. These are are around $400 new and you will be able to photograph more subjects and be available to get into tighter areas more so than you would with the 105mm. Btw, you will likely end up with both sizes.
Tamron also offers a tweener focal length for Nikon bodies that is a very good lens too. That would be their 90mm f/2.8. I know a few people who own that lens and are very pleased with it. Probably not as sharp as the Nikkors, but still a good choice.
Speedflash:
You have two good choices (three counting the new SB900). SB600 and SB800. They are about $100+ difference, but the extra money is well spent for the SB800. It comes with a couple added accessories, but more importantly, it can be used as a "Master" to control up to 3 groups of other speedlights...remotely. You could literally set up around nine flashes wirelessly and be controlled by the SB800 when it is attached to your camera in the hot shoe or with a cable. It is also more powerful than the SB600.
I have been recommending the above initial investment to many folks...including Pat, Mike, and Kevin. It all depends on how much you want to spend for a body, lens, and flash. But, I guarantee that you will not be totally satisfied with your fish pictures until you get a good macro lens and speedflash that is compatible with a decent camera.