Yeah, I have gone thru a few, the one I have now will be the last. I also started him off in a 75 gallon and was eating but just wasn't happy. One day I just said F it, if your not going to make it, I dont want him to die in the small tank.. So I added him to the display and he turned it all around. happy and healthy now!
I think they need a ton of flow to thrive. Not sure what the current is like in your 75. Maybe that is what did the trick.