Monster Tank (Let's Learn Fish)

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Oscars: These fish are legendary for their aggressiveness (they more or less deserve this reputation). They often are brutal to their fellow tank mates and should only be kept with very tough fish. Oscars should be kept alone in a 70 gallon or even better have a small group of them (6 fish) in a 400 gallon aquarium.
Despite their daunting space requirements, these fish are very easy to keep. They will eat almost anything; from flake food, to feeder fish, and even that weird ant on your deck ( I don't recommend feeding them the ant, however). Keep the water at a proper 74-82 tropical fish range.

Red Devil: This particular cichlid deserves its name. This fish is aggressive (like the Oscar) and should only be kept with very robust, aggressive fish. This fish has the same space requirements as the Oscar and has the same food requirements. A group of red devils might work (Note that I stress the "might"). A male and female pair would be best. This fish is probably one of the hardiest fish in aquarium trade. When on vacation, leave an automatic feeder, but make sure large pellets or other heavy duty carnivore food won't clog it.

Clown Knifefish: If you're ever going to see a knifefish, it'll probably be a clown knifefish. This large and eerily beautiful fish is always a fav among monster fish fans.
DO NOT skimp on the tank size! I've seen a great deal of beautiful clown knives in tanks where they can only swim a foot before having to turn around. A tank size of 100 gallons or up is necessary. They can get above 2 feet in length.
Keep them with large fish but keep in mind they can be bullied by especially mean tank mates as they are semi-aggressive.
Feed them live black-worms, and various carnivore food. Worms are essential to the diet of all knifefish. Just watch the blackworms carefully to make sure they don't mess up the substrate (they have a tendency to burrow into gravel and degrade your water quality). You can feed them frozen bloodworms, as a slightly safer alternative. Again, keep this fish at a proper tropical temperature.

Jack Dempsey: Well, there's not much to say about this fish that wasn't already said about the oscars and red devils. Another beautiful but aggressive fish. It basically has the same requirements as red devils (maybe just a tad less aggressive).

Red Tailed Catfish: And (drumroll) the greatest fish of them all! Well maybe not. The red tailed catfish is one of the largest freshwater fish you can keep in captivity (arapaima gigas excluded, as it's really not an aquarium fish). This fish is the unfortunate victim of the "red tailed catfish" syndrome. What happens is some poor, ignorant aquarist goes to the fish store.
That same aquarist sees a cute little red tailed catfish four inches long.
That same aquarist acts on "cute alert" and buys the fish.
That aquarist puts it in his/her 20 gallon aquarium.
That aquarists watches the red tailed catfish grow, filling the entire fish tank up. Don't make this mistake. If you are getting a red tailed catfish, get a tank of at least 300 gallons. These fish get 4 feet plus. Feed them the biggest bottom feeder pellets you can find or ask the fish store.

Black Pacu: Pacus are very (VERY) large fish and are always a staple in a "Monster" fish tank. The Black Pacu is the largest of the pacus, and will get five feet in length. A tank size of 500 gallons is needed to house this fish. When scared they dart around quickly for a hiding spot (so give them hides and plants), but a five foot fish darting around a tank can mean a lot of water on the floor. Make sure your top can resist this. In nature they eat fruits and almost anything that falls onto the water. They will investigate almost anything as a food item. Feed them foods including grapes, peas, cichlid pellets, live foods, meaty foods, cherry tomatoes, and vegetable matter. Slightly acidic P.H. is recommended, so it's a good idea to keep them between 6.5 and 7.5 PH. They are tropical fish.