I have a 12" wild caught male RD who acted skittish for the first week. Not sure how long you've had yours, if you've changed his set up much, etc. My fish shares a 75 with a couple danios and a 2" Synspilum and seems to be content. He will hide when I first walk in the room but comes out once I get close enough for him to recognize me. I just spent 10-15 by the tank every day and he got used to me. He will get really active when the only light on is the hood light. I think this is due to his own reflection showing better on the inside of the glass. At that point he seems to enjoy attacking himself. With that in mind, maybe a tank mate is in order for your fish.