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Piranhas, like the vast majority of predators, mostly attack animals smaller than they are.

Still, I wouldn't say an Oscar is safe in a tank with three piranhas.

You can put almost any combination of fish in a tank for a little while. That doesn't mean it will work out long term.

To compare the two fishes, first I would say that they belong to different fish families, with major anatomical differences. Piranhas are Characins and Oscars are Cichlids. If you look up information on these families, you can find out much more about them.

Secondly, the main thing to know if you are thinking of keeping them as pets is that while both require large tanks, Oscars are more interesting and more fun to keep than piranhas in almost every possible way. Oscars are very intelligent fish and their behavior is complex (for a fish's), and you can even teach them to do tricks.

Because oscars get so big the piranhas see it as an enemy and won't try to attack it unless it's bleeding or something. If the Oscar was the same size as the piranhas then they won't attack it. But if the Oscar was smaller they'd made a meal out of it.

Oscars- Big happy dog like fish that love to eat everything.

Piranah- Pack mentality, paranoid nervouse, can be agressive when it comes to food..

They are some what similar yet they are not.. They both are carnivourse, but oscar arent all that agressive they just eat anything small enough to fit in its mouth. Honestly i would not put them together..