Hi there...I have a demasoni who looks like he is in very bad shape

He used to be the dominate fish, but ever since my Auratus matured the demasoni has gone done hill. I have 6 mixed species of mbunas in a 42 g tank and my water levels etc are great and the tank has good cover/rock caves. The other mbanas who are a bit larger than the demasoni seem to be in great shape. I did some reading and saw that the Auratus is the most agressive of the mbunas and needs 125 g tank. I think I'm going to trade the Auratus back to the pet shop and get others.
But, any tips on nursing my poor beat up demansoni back to health? He seems to be a lot skinner, I never see him feed, and his jaw looks a little crooked or something
