Taking pictures of fish in aquariums is not easy, there are
lots of factors to be taken into account:

1. Glass and water to shoot through
2. Fish that keep moving (!)
3. Light reflected off the glass

4.Delayed reaction of digital cameras

Etc.

From my limited experience the best way is by using only the aquarium lights
and getting the front of the lens as close to the glass as possible; try to
focus the camera on the subject fish, and use as wide an aperture as you can
to eliminate dirt and such like on the front glass of the aquarium.

I saw mentioned in a photography magazine recently that the best way they
had found was to use an off camera flash gun resting on the plastic lid that
protects the aquarium lights from the water - no idea how well that would
work though. I might add that some fish may be sensitive to flashes so                                    observe them for the first time.

There are a couple of sites on the web that might help:

http://forums.aquariumhobbyist.com/forum.php?catid=5

http://web.mit.edu/lxs/www/cichlids/fishphotos.html

Thanks to Tom Horsley for his advice http://www.thorsley.co.uk/

All of the photos below were taken with a digital camera costing less than £100. I would suggest that only take photos of fish that are not moving. I found that using the flash does bring you better results.  However, you have got to take into account reflection from the actual Flash itself so I have found that taking the picture at an angle can help.

Below are photos taken with the same camera but this time using the flash