@Summer Tanager Thanks! The main tricks are to 1) hold your camera exactly parallel to the fish when it swims near the front of the tank, to overcome distortion, and 2) move your camera at the same speed the fish is swimming.
@RatioTile That's great advice! Holding the camera parallel is tricky in and of itself for reasons that I can't explain; but moving at the same speed as the fish is even harder! Thanks for the tips!