Well, there are lots of tips for taking pictures. First, pic and object in the tank that you can focus on, lock your focus on that object then move the camera back a few inches. Now, wait for the fish to swim infront of the focused object and snap the photo. You should now be correctly focused for the fish.
Other factors to take into account include tank lighting and back lighting. Really. if you have enough tank lighting you will not need to use a flash, however if you don't then the image will take too long to process and you may get fuzzy pictures. The camera basically keeps the shutter open until it has sufficient data from light to create an image.. the longer the shutter is open the more chance your fish will move.
Try to mount a few lights above and below the tank to highlight the fish and not the glass. This also helps show the true color beauty of the fish.. Hope that all helps.