Some tropical fish, either because of their size, feeding habits, or just their natural behavior, are not suitable for beginners to try and raise in a community tank. These are some of the big and bad, that you may want to avoid until you're more experienced.
Oscar- This native of the Amazon River and its tributaries, is a large fish, reaching 13-14" in length, although it can be sexually mature and laying eggs at 4". They do best in a tank with no "fussy" things like slender plants or ornaments. They prefer a medium texture substrate because they're great diggers, but do like wood or rock platforms that create a cave. The recommended food for Oscars is feeder goldfish, because they basically will eat anything small, that moves. This is why they can't be kept with smaller fish, or livebearers that will have young. Most hobbyists use a special large stick food that absorbs some water and moves with the motion in the aquarium, so it mimics prey.
Jack Dempsey- Another South American native, the Jack Dempsey comes in many of the same dark colors and spotting as the Oscar, featuring greens, brown and gray areas, which may help large species like this to hide amongst the bottom rocks. The Dempsey is similar to the Oscar in other ways as well, being a bottom digger, and preferring caves and wood to rest under. They are also a live feeder that will devour anything that moves, but unlike the Dempsey, they pursue their prey, and are considered to have "attitude" that makes them best suited to a tank of like-minded fish.
Discus- While not the bottomless pits that Oscars and Jack Dempseys are for feeding, the Discus is still a large fish, even at 6", and because of their native Amazon River environment, require a fairly specific habitat. In the wild they lived where trees had fallen into the river, and made their homes under and around the branches. In an aquarium, that means keeping a thick substrate where the light does not reach down to, as well as lots of wood pieces for hiding, and vegetation that goes from bottom to top. They are live feeders as many large fish are, but generally subsist on a diet of shrimp, tubifex and daphnia in good quantity. They are a fish that lives naturally in groups of five or six, and in the home environment, do not take well to upsets or changes in the tank.
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