Some tropical fish, whether due to their size, eating habits, or simply their natural behavior, are not suitable for beginners to raise in a community tank. Here are some of the big bad ones that you might 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 lay eggs at 4″. They do best in a tank with no “picky” things like spindly plants or ornaments. They prefer a medium-textured substrate because they are great diggers, but they like wooden or rock platforms that create a cave. The recommended food for Oscars is feeder goldfish, because they will basically eat anything small that moves. That is why they cannot be kept with smaller or viviparous fish that will have young. Most hobbyists use a special large stick food that absorbs some water and moves with movement in the aquarium, thus mimicking prey.
Jack Dempsey – Another South American native, the Jack Dempsey comes in many of the same dark colors and spots as the Oscar, with areas of green, brown, and gray, which can help large species like this one hide among the bottom rocks. . The Dempsey is also similar to the Oscar in other ways as it is a bottom digger and prefers caves and wood to rest under. They are also a live feeder that will gobble up anything that moves, but unlike the Dempsey, they chase their prey and are considered to have an “attitude” that makes them more suitable for a tank of like-minded fish.
Discus – Although not the bottomless pits that Oscars and Jack Dempseys are to feed on, Discus is still a large fish, even at 6″, and due to its native Amazon River environment, requires a fairly specific habitat. In The wild lived where the trees had fallen into the river, and they built their homes under and around the branches.In an aquarium, that means keeping a thick substrate where light doesn’t reach, as well as lots of pieces of wood to hide in and vegetation that goes .bottom up.They are live eaters as are many large fish, but generally subsist on a diet of shrimp, tubifex and daphnia in good quantity.It is a fish that naturally lives in groups of five or six, and in the domestic environment . , do not get along with rollovers or changes in the tank.