Thanks to all.
All fish are shot with flash. It's the only way to "freeze" the motion. I'm using a 135L lens with a 12-mm extension tube on autofocus and a diffused flash on a Wimberley macro bracket. Flash is a Canon 580exII unit with a StoFen diffuser. Camera is a 7D set to AI Servo mode. Most cameras cannot track fish in any autofocus mode, so you'll have to manually focus.
A much more economical way is to mount a flash unit on the camera hot shoe and place the head at an angle that approximates 45 deg. Attach a white card to the flash head. Set your camera at ISO 400 and f/8 or f/11. Use the fastest shutter speed your camera can deliver and be in synch with the flash. Put the flash in manual mode and adjust the light output to get the proper exposure.
Get a chair/stool and sit down. Wait for the fish to come to a spot, then take the picture. Chasing them up and down the tank will just make you tired.
Gary