I wouldn't go below 72 degrees F with any tropical fish. I've had several different species of of suckerfish, and they seem to do great at high temperatures(80-83) with discus. Exactly how cold is the tank? Intank heaters are only around $8-$12, if you go cheaply. Really, tropical and coldwater fish should never be mixed, because one will always be living in an unsuitable environment, whether it's the tropical one, as in your case, or the coldwater one.