you should probably go visit an aquarium specific photo forum. i know there are some around. there are many experts on there who can advise you on the proper equipment to use.
as a point of reference to you, it's going to cost a lot more than your budget for good equipment. the camera body usually runs at least $500 for the latest versions, and a good lens will set you back around $500 as well. the flash, especially the 'slave' and 'master' set ups i.e. those flash units you mount on top of the tank that are controlled by a remote unit mounted on your camera will cost you i think $300++
photography is all about capturing light. i personally would recommend a better lens and flash unit set up as opposed to an expensive camera body. camera bodies are much more prone to price depreciation as a result of how fast the technology in that area changes. for example, i got my camera body for around $500 around 2.5 years ago. i bet now it's only worth around $350 or something. on the other hand, i got a really good lens for $500 and i sold it one year later for $450. the depreciation does not go down much for good lenses.
all the best in your search.
