In my experience it actually doesn't matter what kind of camera you have, it's how you use it really (unless it is a camera phone)
The thing to be aware of is cameras work by letting light in..the longer you can let light in the more detail your image is going to reveal.
THat being said, you have to keep the camera steady with a longer exposure or the photo will be blurry.
The best setting for photos is a high resolution without flash AND a tripod to put your camera on to keep it steady (no blur)
You can improvise by making your own tripod out of anything really, (a stack of books for example).
Rest the camera on your "tripod" to keep it steady while you take the photo.
This way you don't need to use our flash, which will reflect of any reflective surface and mar your image quality.
Also I have found cleaning the outside of your tank, and putting the lens right up to the glass to be effective.
Turn on any other sources of light you may have in the room and go crazy.
Quantity is important,often only 1 out of 100 shots is truly great, so the more you take the better luck you have!
I also like the "zoom" setting on my camera for fish pictures. The other thing to remember is your camera has to focus on where the fish is, not the back ground or foreground, or else your photo will be blurry. However it is hard to focus on a moving object, so I often focus on my guy, and keep the camera settings in place until he swims by again and take the shot.