Very nice midas/RD hybrid. I would say +1 to Justarn, if you have a large 200+ gallon tank these guys could probably live together if there were 3-4 other large fish to spread the aggression out. If you want to keep them alone, 75 gallon is about the smallest size they will be happy in long-term.
Some tips about the feeding- fish eat when they feel comfortable, do you have any décor in the tank? If it is bare I would try adding some fake plants or flower pots. If you aren't seeing any progress in a week or two, try soaking the pellets in crushed garlic. This will stimulate the appetite, I have had luck with this.
Best luck!