If the tank is empty, just replace it ... you could potentially replace the broken side of the tank by cutting the silicone seal around that pane of glass to remove it and then get a piece of glass cut the same size to replace it with (the tough part will be removing the plastic frame without breaking it). If the tank is in use, a temporary patch could be made by using silicone to hold a piece of glass over the crack (do this on the inside of the tank).