Well my pair of cons is in a 20, and since I decided to keep this batch of fry in the tank, the female is trying to eat them. My male has completely taken over all parental duties and hardly lets my female out of her cave. I have them and 2 zebra danios as dithers in a tank. I think these little spats that they have is because I didn't allow my male to choose his mate, I chose her for him.
But cons will breed anytime and anywhere. Try throwing a few dither fish in with them and see what happens. If they see a few fish swimming around like chickens with their heads cut off, they will feel a bit more comfortable. Maybe a few zebra danios or tiger barbs. Did you purchase the pair separately, or did you buy a handful and let them pair off?