If you keep single fish tanks with non-aggressive fish like me (goldfish), employ a small lightweight hand mirror stuck to the outside glass with tape on the side of the tank. That way your lonely fish will receive some stimulation when your not there. You could also use it for aggressive fish as well, as long as it doesn't stress them out too much. For example in the case of an oscar or midas, small plastic bird mirrors suspended inside the tank may provide hours of stimulation.
