Megapixels dont really matter (unless you're making huge prints). I shoot with a 5mp camera. You need a camera with a quality CCD and lense. Usually the thing to look for in aquarium photography is a camera that is capable of shooting at a high ISO with little noise. The biggest secret is to load the tank with light, shoot with as slow of a shutter as you can without it getting blurry, and being able to control your apeture settings. A slave flash works well also.
I believe that in digital cameras, you get what you pay for.