here's some things to try:
take the hood / canopy / lights off your tank, and put a few 100 watts of halogen into it (any shutterbug should have a few halogen floods laying around!)
the halogen will give you enough light to focus, and maybe do a few slow shutter shots.
next, with a tripod or some clamps, mount a flash or two either directly above your tank's surface shining down and in, or mount the flash at a 45 degree angle to the glass.
when using a flash, make sure your glass is SPOTLESS otherwise it will show in your flash shots!
also, when using a flash, make sure there is nothing around the tank to reflect light back at the glass, because the glass will also act as a mirror. use black felt to matte your stand / wall behind the tank. (i got two yards of black felt cheap from a fabric store)
hope this helps!
I've yet to figure out how to macro shoot my fast fish ... if it is even possible.