Some basics while shooting through glass (and aquariums): off-camera flash (that gets rid of the flat look), use a high vantage point (as Kami said) - that give a more "natural" look and shadows, use a polarizer (eliminates reflections from the glass), use a tripod and manual focusing (don't expect the camera to know what you're interested in), pre-focus and wait for the fish to move into position, take several exposures and see which ones give you the best over-all rendition, then set that exposure in manual so that your exposure doesn't get influenced by the bright or dark fish moving around. Good aquarium photography isn't easy.