Thanks man!
I'm really a novice and only use the the T1i, no special lens and no external flash. All my pics are taken in manual focus and with the flash turned off.
Since I don't use a external flash I use high ISO settings, usually 1600 or 3200.
As for shutter speed it depends on which tank I'm shooting and the fish. Some of my tanks have dual light setups and others have single bulb setups. Since there's more light on the dual light tanks I use a lower ISO and on the lower light tank a higher ISO.
The same goes for the shutter speeds, on the lower light set up I use a slower shutter speed and on the brighter light tank I use a faster shutter speed. Usually on the lower light tank my shutter speed ranges from 1/20, 1/50 and on the brighter light tank a shutter speed of 1/80, 1/125. The numbers also change based on how cooperative the fish is. Some come up close and stay toward the upper levels(expecting food) when I'm shooting and make taking pics easier. While I have others who stay low in the tank and are very darty. Of course the more calm the fish the lower a shutter speed I can use while the more active the fish the higher shutter speed I have to use.
Again I'm just a novice and my settings aren't perfect but without an external flash they might be a good place to start your own experimenting. I hope my amswers help.