if the problem is overall focus and not because he i moving, plan on how far away he will be when you snap the shot, aim at something at the same distance that is outside of the tank, half press the shutter button and this should lock focus so that when you put your camera up against the glass, it will focus up to as far away as the object you focused on outside of the tank. Follow the fish around until he gets within that range, and BAM you got the shot. Worth a shot.
Mark