here's a few things that I know help make shots good
#1 Light - Lots and lots of light! The normal lighting for your tank might be enough for viewing the fish, growing the plants or feeding the coral but it's sometimes not enough to capture a fast moving fish or brilliant colors on a high resolution camera. My I'm trying to get "the shot", I often suppliment my tank's normal lighting with several 100 watts of tungesten halogen lighting. Using plain old $7 outdoor floodlights and $8 clip-on fixtures, I can highlight things and bring a lot of extra light to the tank, without having to use the camera's flash. Halogen also has supurb CRI or color rendition index, rivaled only by bright sunlight - this means all of the colors of your tank will stand out and look their greatest on film.
#2 Settings - Things like shutter speed, lense aperture and iso mode all have a bearing on how your pictures come out. There are also settings like exposure compensation and white balance but they take a back seat to shutter and aperture. A fast shutter freezes action, but it also gives the camera less time to record an image. Adjusting the aperture of the lense allows more light into the camera and can make up for a fast shutter, at the expense of depth of field. The ISO can also make up for a fast shutter, the higher the iso the faster the "film" records light, at the expense of accuracy ... inaccurate recording of light leads to grainy pictures.
#3 Patience - digital photography is very inexpensive ... keep shooting, change one little thing and shoot another dozen shots ... you can always delete what doesn't turn out - another invaluable tool is a tripod or monopod ... using a slow shutter can result in beautiful pictures, but only if the camera doesn't move while taking the picture
Hope some of this information helps you!