Given how dense and thick a pacu is, a 3 ft tank is definitely not suitable. Plus they get way too big for any "normal" size aquarium anyway. To be frank, it might be best to rehome the fish. Getting smaller similar fish like scats or silver dollars would be my adviceI mean some foot long guys can work in a 40b, just they are all long bois, like the smallest of the Bichir, Ropefish, and Eels.