I made up a small background/bottom out of white water color paper. The white paper helps brighten the background and reflect additional light into the tank, onto the fly. A couple small mirrors are also handy. The whole setup is less than ten bucks. Easy to set up and adjust. Easy to store. And a fun way to play around, while learning how fly patterns behave on/in water, and look like underwater.
Basis Set Up (Tank/Paper Clip/Fine copper wire):
Fly held in place with small binder clip. Quick and easy adjustment of fly position.
The end flaps can be bent backwards/forwards to allow better viewing angles (don't forget the end of the tank).
Placing the fly at the far end, allows reflection of the fly to show different angles in the near side.
Depending on viewing angle, you can see multiple images/angles of the same fly.