I've got a Kodak compact/mini DSLR, and I've found that an ISO of 64 along with a shutterspeed of 1/60 - 1/250 (for faster fish) works best. The trick is light - for the best quality pic's you need good quality lighting - ideally turn the flash off and use your tank lighting, and cut out light sources from other directions.
A tripod also helps, especially when using lower shutter speeds. And always always always use the timer if your camera has one - set it to a 2 second delay so that the pressing of the button doesn't wobble the camera.
If you don't have a tripod and can't use a table or similar, crouch down (depending on the tank height) with one knee on the floor and if possible lean against something to make yourself as stable as possible. Then use the viewfinder on your camera - this makes you bring the camera in close to your body, keeping the camera stable. If like me you've got an SLR or compact SLR cup your left hand under the camera, wrapping your index finger and thumb around the edge of the lens.
That should give you a pretty stable platform.
If you're taking photo's during the day, near a window then you're going to struggle though - as I've been finding out recently. Its really difficult to cut out the reflection from a strong light source (i.e the window during daylight.) The only way I found that works is looking at the front glass of the tank, work out where you can see the reflection of the window - and move your body so that its not visible where you're aiming to take the pic of.
I'm no expert but thats what works for me.
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