I agree with what's been said, but I'll add on a bit. First, I'd find the basic camera body type your happy with. Both Canon and Nikon have a larger body type with more functionality at your fingertips (i.e. more knobs and buttons) and a smaller body type (known as the "Rebel" line for Canon) that tends to be less expensive. At a minimum, I'd try to get my hands on the two biggest players out there, Canon and Nikon. Both companies make very comparable cameras, so don't worry about quality at this point, just how the camera feels in your hands and how easy it is for you to go through the menus and change camera settings.
Once you've decided on a brand, take a look at the lens(es) that you'd like. For aquarium macro photography, I'd recommend a true macro lens around the 100mm focal length. One warning, if it's a zoom lens, it's not a macro lens despite any "macro" nomenclature attached to the lens's description. But the lens is where you want don't want to skimp on money, so do some research to find what you really want. Here's a good place to start that research:
http://www.the-digital-picture.com/
Now, figure out how much money you have left in your budget after buying the lens and really nail down what camera you want. I'd say choose the camera second, because quite honestly, every dSLR on the market as well as the previous couple generations are quite capable of very high quality photos in the right hands. Don't forget about used cameras as well. Online compainies like B&H or Adorama have used departments with a rating system on the camera's quality that should take a lot of the fear out of buying used if you decide that's something you'd like to do. And it might be, because we haven't even discussed tripods yet. . .