What your red devil is trying to say is, "Set me free! This tank is too small for me!" You can do your fish a major favor by finding a nice nearby lake, bond, creek or river and setting the little guy free. It's for the best. It is a decision that you will definitely not regret!
JUST KIDDING!!!
I'm having the same problem. I'm in the process of setting up a 225 gallon tank and the first fish I got was a nice baby Jag, then 2 baby GT's, A mexican tailspot, a Red Devil and a real nice Flowerhorn. All baby's. Less than 2" apiece. I'm keeping them in a 55 meantime. The Red Devil got really aggressive so I put him in an isolation tank about 5 gallons that hangs in the 55. I drilled holes in the 5 gallon myself so that he would be getting plenty of fresh water flow going in there. After a few days I thought maybe he got the idea and I released him back into the tank. My wife and I left for about an hour to go get some supper and we got back to find one of the GT's dead and the other one well on it's way to death. Both his eyes had been eaten out and he was badly beaten. The aggression continued for a few days, not as bad since the other at least can hold there own. Finally I put the Red Devil back in isolation. Once the 225 is a go, I am just going to let him have the 55 or maybe my 30 all to his self. What an aggressive bastard! He's a beautiful fish though. I guess he's what's known as the Tiger striped varient. He's got great black banding on a nice red and grey body. Beautiful fish. Already getting a Nuchal hump. I do have one question though... Why do they call them Green TERRORS when they seem to have about the same aggressive tendencies as say a... I don't know... Gold Ram?!