Roomate had a red devil, who every so often would decide to pick on the other fish. To attempt to reset the terroritories my roomate would rearrange the tank when this happened, before finally giving up and trading it in... As a result the fish went from begging for food at the top when he opened the top, to hiding under the filter intake on it's side instead when he got near, and learned to do this within days,and would only eat if the top got closed again, so theres a trick it learned without having food as a reward, although it's more linked to survival instinct then anything else.
