Get a tripod, or similar, and rest the camera on that. turn off the flash and all other lights in the room, so it's as dark as possible, with only the tank lights on. Take a few pictures of the whole tank, and center in from their. This is how my better shots have been taken.
can i suggest a forum search? there have been many threads on this subject. not all techniques, suite all people. give the threads a read and refine your own method, based on what you learn. when it comes together, it is an amazing thing. all the best!