You're going to need to spend 600-900 on a good macro lens. You can get away with a D40, D40X, D80 (or Canon equiv) camera for 400-1000$. If you get a good general purpose lens you can get the camera, a good lens and a set of extension tubes for 1500 or less. That will get you to the point where you'll have to hone your skills.
Any 1/2 decent point and shoot should take nice, well focused pictures of your tank. If it isn't then you need to work on your technique before any camera will produce good pictures for you (DSLR or not).