Moved to Show Off/Photography, since this is a topic about taking pictures and not a question about a specific fish that is pictured. (make sense, lol?)
I am not an expert at all with digital photography, but I've noticed some improvement in my pictures the more I take them. If they aren't any good, just delete and try again. Practice does make perfect in digital photography! Make sure you know all the settings that your camera is capable of. Experiment with and without flash, or with manual settings vs. auto settings. I've re-read my owner's manual a few times to fully understand all the different settings that it has. And like Rich says, take lots of pictures and you'll get a few you like. Just keep trying - sometimes the fish is just not in the right area of the tank to show up well, so wait a bit and try again.