What size is the tank? And as suggested do a full set of water quality tests. 99% of all problems in the aquarium stem from water quality unless you just happened to start out with a diseased fish from day 1. Those two and the pleco should be in a minimum of a 20 gallon tank, and ideally I would say they should be alone without the pleco. So if the tank is smaller then that it could be stress mixed with water quality. Because he used to be alone he never really concerned himself with breeding to any large degree, but now with a female present if he cant provide a secure nesting site, it may be stressing him quite badly.