I've kept singles many times. In fact I have a Bolivian male in another tank and recently added a female and they don't like each other! When they meet they act like two males and then each retreat to their half of the tank. The females belly is turning pinker and pinker and they still snub each other. So the point of the story is even when a potential mate is added doesn't mean they will accept each other. I think my male actually prefered being the only ram in the tank because then he had a larger territory and didn't have to share any of it.