Without knowing your specific camera, it's tough to give specific advice.
The best general photography tip I've ever heard was; "If your pictures aren't good enough... you're not close enough."
This is a quote of renowned photographer Robert Capa's. It's interesting advice coming from a war photographer. A war photographer who subsequently died by stepping on a land mine, 'getting close'.
http://en.wikipedia.org/wiki/Robert_Capa
Luckily there are not that many land mines in aquarium photography. To create photos that have more impact, choose the subject you are creating an image of and feature it by filling your camera's frame with it. Get close, zoom in or crop in after-the-fact.
Also keep the 'rules of composition' in mind while filling your frame (Google them, there are many).
Cheers, Chris
