When you're putting expensive tropicals out in a pond, having them get eaten by varmints, birds, dragonfly larvae, etc is a problem. Getting fish or fry out of a pond at the end of the season isn't easy, either.
At club auctions, I've never seen catfish go cheap. Killifish and dwarf cichlids (except for kribs and shell dwellers) always seem to get good prices as well.
great points, i have been thinking about that also. Maybe I will have to get fish that are big enough to eat the wrigglers (Aquatic dragonfly larvae)