Linda, you'll need a much larger tank to house your Red Devil - at least 55 gals and preferably 75. And this is with no tankmates.
Though they are carnivores, they do need a varied diet including vegetables like spinach, peas etc.
As Dino said, certain setups are needed for breeding to avoid the female being killed, so unless you're ready to set up more huge tanks for breeding, growing up fry, and refuge for the female, I would just keep your one Devil alone as a pet in a very large tank!

These intelligent fish are very interactive and entertaining.
Agree with Dino, though. Some species of fish should not be sold in petstores.