Male not coloring up anymore

I have a unknown zebra that is about 2 years old. He is the dominate male in my 75g but ever since my tank got sick about 3 months ago and I lost 20 fish, he's stopped coloring up. I don't know what illness the tank got. Ick treatment did not do anything so I turned up the temp and added a lot of salt to the tank and all the fish got better immediately. The fish that I lost did not look sick. The ones that looked like there were going to die, lived. He still colors up when he gets mad from time to time but then looses it.
He's gone from the first pic below to a completely purple color like the last pic. What could be the problem?

Colored Up: Fish turning


After tank got sick: Fish on the bottom

He never had any females because I don't know what exactly what he is. The only difference I can see is he's grown, 4inch in the first pic, 6-7inch the second.

I guess I can only hope he one day goes back to his original color.

He's still the dominate male. He has control of most of the caves and chases all the other fish on a regular basis.