I recommend checking craigslist for a dSLR. Much like reefing, people are always getting out of the hobby or upgrading to new equipment.
The ISO of a camera is important. Especially on dSLR bodies. The higher the number ie 3200+ the more sensitive to light your camera will be. On dSLR bodies you pay a premium for this technology. This is beneficial for a few reasons. First, it will allow you to have a faster shutter speed freezing movement in your tank. Second, it typically reduces overall noise in your photos because better camera perform better at higher ISO's. You want to try to keep your ISO as low as the situation will allow.
On lenses for dSLR cameras you pay a premium for the quickness of a lens. This is measure by a F stop number typically in front of the lens range. IE 1.8 50mm. The lower the number, the faster your lens, and the more light it will gather. Again, this will allow you to keep your ISO low reducing your noise.
If you want to work with the equipment you have try building a photobox. Basically its an acrylic box that will allow you to submerge your camera body. The macro pics I have seen from others always seem to turn out top notch.