Another thing most people don't know, is that even though an aquarium looks bright, it is actually pretty low light for a camera. Even under metal halides. When you zoom in as with most macro shots, the light is even reduced more. The camera in auto modes adjust for this by slowing the shutter speed and opening the aperature. Now you have motion blur and DoF problems. The flash can fix this, but you run into problems there too. If the flash is to bright, it'll blow out your highlights, if it flashes at a differect color temperature then it won't look like you see your tank, and in extreme close conditions, the lens can shade the subject of the flash can shoot its light over the top of the subject. Oh, and the flash can reflect off the glass.
SO don't get all upset if your having trouble. It isn't easy. The good thing about digital though, you can shoot thousands of pictures for cheap untill you figure it out.
Mike