Although there is some truth in this post (as far as color, at least), it could not be farther from the truth in its intended context. Red belly pacu (Piaractus brachypomus), and Black pacu (Colossoma macropomum), are not even in the same genus, let alone the same species. And btw, C. macropomum seem to be quite rare these days; if you've bought one, you will know it.
OP: unless you have the means to build/buy a massive (800+ gallons) indoor pond/aquarium, get rid of that thing ASAP. It will outgrow anything else and will be a nightmare to care for in a few years. And it will be much harder to get rid of as it grows.