I never like to use flash, just cause my fishes always seem to spazz and look dazed when I do, though I don't know if it actually hurts them.
On your camera, you should have a option that's like a little flower (macro), and that will help the clarity. Also, never zoom in your camera, it makes it just look fuzzy, at least on my camera it does. Always just move the camera in close. I've tried artificial light, but just never looks as good as good ole' sunlight. I usually just open the blind so the sun hits the tank for a bit and take some pictures.
It also takes a lot of patience lol