I have a Canon Powershot 780IS. Mine has a Kids&Pets mode, but it isn't fast enough for my fish =) The auto mode honestly works pretty well, EXCEPT when my fish won't stop moving >_> However! There are a few tricks you can do to get better pics.
* Set the mode to Macro (close-up) - this is the left-side button on mine
* If your pics are dark, turn up the exposure - this is the down button on mine
* I turned off the autofocus servo, because it kept wiggling the focus.
* If you hold down the take-picture button half-way, your camera will try to autofocus and figure out exposure/shutter speed. If you point your camera at a light (or something bright) and start the auto-focus, it will typically come up with a fast shutter speed (1/100 plus) instead of the usual slow one (1/8, 1/25). Then you can swing around to your fish and normal lighting and take a fast picture. You can also try autofocusing on a white piece of paper or the wall, etc.
* Play around with the auto-focus computation algorithms. If you use the point-computation instead of evaluative, I've managed to get it up to 1/800. The faster shutter speed will let you take crisper pictures when your fish is moving, though you will need better lighting. (You might also want to turn up ISO since you are getting less light.)
Here are a couple of guides that have helped me figure out some of my camera settings!