I have an R.D. that is incredible. He likes to hide, but when it comes to hunger, he is all over the tank swimming and waiting for any morsel to drop in. I have successfully kept him in my tank with two parrots, one young oscar, 2 med size pacus and a couple of plecos. This red devil lived in my outside pond all summer with the Koi's. He seemed to like it out there and definitely ran the tank. It works out in the summer time here in California, but the winters get to cold for our Central America native fish. Mine is about 18 cm but still kind of lean. My brother's red devil is 23 cm and is very adult looking...plump with a huge hump and bright red color. He keeps his with a 60 cm silver arowana, 13 cm Texas, and a 20 cm frontosa. They all get along pretty well. So it is true that you can keep red devils with other fish. Keep in mind it doesn't always work, and if it does work...the question is how long will it work.
My wife and I bought a mystery fish for $5 at a pet shop. It was about 3 cm long, very pretty, and had plenty of room in the 20 liter tank we had. Of course, Yellow Fish turned out to be a Red Devil, and quickly outgrew her tank. We got her a 220 liter tank, but she considers the entire room to be her territory. If my wife or I, or one of our cats for that matter, approaches the tank, 22 cm Yellow Fish immediately confronts the intruder, only backing away when the other leaves. On occasion, in the interests of science, I have stuck a fingertip in the water for her consideration. What I got for my trouble was no curious nip. She clamped on for all she was worth and did her best to tear off a piece of my flesh. She is definitely the smartest fish I've ever seen. Even in those first days on the top of our dresser, if she spotted my wife or I stirring in bed, she would leap out of the water and strike the lid on the tank hood to let us know she wanted attention (or more likely, food). To this day, she slaps the tank hood whenever she feels neglected. Sometimes we have trouble figuring out what she wants, but Yellow Fish is patient, and our training continues apace.
I had a Red Devil that was over 33 cm. He was a huge male with a big hump on his head. Absolutely the meanest, nastiest fish I have ever seen. I could not even keep turtles in his tank. He would role them across the bottom of the tank and smash them into the glass. Needless to say he lived in a tank by himself.
I keep a tropical pond where in the winter I erect a dome greenhouse over to maintain my cichlids outdoors year round. It is around 1500 liters. I have numerous cichlids and thought I could offer a different point of view on this species. They are in there with jack dempseys, jewels, mollies, minnows, shubunkins, fantail goldfish, comets, plecos, freshwater mussels, shrimp, and a school of male bettas, yes male bettas and none of them fight. These guys get less aggressive outdoors since they worry about being eaten as well, I keep it very deep for a pond, approximately 115 cm deep, so they can retreat from predators, heavily planted with lots of caves etc. I have never had a dead fish and I maintain a constant count on anything 3 cm or bigger. I can't count the shrimp, but I know I got about 100 ghost shrimp. I assume they keep feed on these and the snails. All the fish get along and my two red devils are so tame that they hand feed without biting and I can pet them like dogs. Not so lucky with the jack dempseys or the jewels, they are a bit more skittish, but the two reds are very friendly and allow just me to pet them. And if someone tries to remove any fish via net they attack if it's not me doing it. They are the self appointed guardians of my pond and protect the smallest fry of any species vigorosly. Anyways I worried about aggression and thought to myself when I first threw them in there "what am I doing", but I have a very tame pond with two of the most friendly fish I've met.
When our red devil was younger, about 7 cm long and kept in a 40 L, he was very aggressive. He had a hiding place, but he would come out every now and then and beat on the other fish in the tank. We were worried he was going to kill them, so we started using a broken plastic shirt hanger to chase him around the tank. We kept the hanger in a drawer underneath the tank. Soon, whenever he saw the hanger outside of the tank he would go hide. Then he got even smarter, and when he heard the drawer open he would go hide! And some people say fish only have a 5 second memory. He is now over 15 cm long and is in a 200 L tank. He now has a new, larger hiding place, but he doesn't use it much any more. As he grew up he completely changed color, from dark with black stripes to yellowish white with some small spots. It's been a great experience to watch him grow, and I can't wait till he's huge!
I've had them for 2 years now. I have two males and two females. They are housed in a 470 liter tank. One set of parents just had fry, I got to save about 40 of them. Very interesting fish. I've been bitten more then once, drew blood every time. They are very good parents, even my other set did not mess with the fry. My biggest male, a few months ago molted. If you have never seen it, it's a sight to see. He turned almost all the way black, but came back firey orange in a couple weeks or so. I made a mistake by putting a few fish in over the time I've had them. They act like piranhas, they attack in a pack. One was a very big Texas cichlid, the other was an albino oscar, may they rest in peace.