I am purchasing a Nikon camera and have questions about what type I need to purchases.?

I order to photograph fish in an aquarium you are going to need at least one of three things.
High ISO
A bright Flash
A fast Lens

The D3000 only goes up to 1600 ISO, not nearly sensitive enough for a dark aquarium.

I moved over to the lenses page and it looks like all of the Standard lenses are f/1.4. Very fast. But they dont zoom, so lets move on.
This is better. http://www.nikonusa.com/Find-Your-Nikon/Product/Camera-Lenses/2164/AF-S-NIKKOR-24-70mm-f%252F2.8G-ED.html Its aperture is a little slower at 2.8 but it stays consistent to 70mm so you wont lose light when you zoom in for fish at the back of the tank. If thats too expensive, this one isnt as good, but might not leave dust in your wallet. http://www.nikonusa.com/Find-Your-Nikon/Product/Camera-Lenses/1929/AF-Zoom-NIKKOR-24-85mm-f%252F2.8-4D-IF.html

The flash that comes on your camera is dinky and wont do much good for anything, so if your aquarium allows flash, I’d go ahead and start looking for one here: http://www.nikonusa.com/Find-Your-Nikon/Flashes/index.page

Good luck to you. =]

Forget the mega pixels, all cameras are fine these days. Whats really going to matter to you is aperture and ISO. Aquariums tend to be very dim a lot of the time, so in order to get proper shutter speeds your going to probably be shooting anywhere from ISO 400 – 3200. This means that your going to need a much higher end camera to avoid lots of noise especially above ISO 800. The D3000 is a low end DSLR and probably wont have very acceptable noise about ISO 800 and does not go above ISO 1600. I don’t know how important these pictures are (for fun? for a job? other?) but depending on the circumstance you may want to look into a much better camera, at least the D300. As for a lens, focal length is anyones guess and will largely depend on what you are shooting at the aquarium. If I had to pick just one lens to bring though, it would probably be some sort of telephoto, because you can always back up, but in this case you might not be able to get closer. However you will want lenses with an f/2.8 or larger maximum aperture, and again, depending on how important these pictures are you may want several lenses and then a really low light lens like an f/1.8 or f/1.4 lens.
Edit: do not use a flash unless you somehow have the technology to use underwater remote flash in your pictures and someone in the tank to position it. The flash is just going to cause you a lot of problems with reflections off the tank glass.
Edit2: Also, learning how to properly set your white balance will help improve your pictures a lot.
Edit3: A polarizing filter will help cut reflections, but its also going to lower the amount of light coming into the camera which in your case may not be a good thing. It might not hurt to have one but in many cases it may be more of a hindrance.

Get a polarizing filter to screw on your lens. It will help cut the reflections from the glass (and water if from above). If you are able to stick the lens to the glass, it would be best to have a rubber hood retracted to prevent scratching the glass.

Set your camera to action mode and take lots and lots of pictures. You may need to bring an extra set of batteries and memory cards. Megapixels don’t matter and the kit lens would be just fine. Avoid using the flash unless you can stick the lens to the glass. Some large aquarium parks do not allow flash.

Megapixels have to do with printing and cropping. 10mp is fine (you can’t find many under 10 anyhow). The lens is what is going to give you good, sharp photos. You want a 50mm that says “F/1.4″

That means the lens’s eyes will be wide open and gets alot of light in. A lens that says F/4 will be like squinting and there will be less light. Then the pictures will be darker and come out blurry.

You want to ask the guy at the camera store for a “50mm, f 1.4″
Or impress the guy by asking for a “nifty fifty.”