I usually use the flower setting and make sure the fish is at the centre. Also, diable flash if the tank light is on. Otherwise, try takng the pics at an angle to the glass so that the flash does not reflect back to the cam.
I use to when I had a manual camera, now I have a digital and set it on Potrait for tank shots, and Micro for close-ups. I also set the camera on the max resolution (7.1 in my case).
Personally, the thing that helped me the most was using a Tripod. Anything will do - I've used a chair/TV tray more often than not.