This is NOT true. A single oscar will need at least a 75G tank to be happy. Oscars are messy fish. They eat alot and poop alot. If you were to put one into a 5G tank, the water quality would be terrible. The oscar would become stunted as well. Although some say that stunting only causes the fish to stop growing prematurely, this is not the case. With stunting, the outside of this fish will be smaller than normal, however, the internal organs will continue to grow resulting in a grossly deformed fish. This is a horrible life for any fish, but a 5g tank for an oscar is unthinkable. PLEASE PLEASE do not put an oscar in anything smaller than a 75G.