I had a Red Belly Pacu in a 29 gal. tank for years. They tend to grow depending on the tank size, at least, that has been my experience. Mine grew about 13 inches, was very active and healthy. I had to move and gave it to a friend. It lived with him, in the same tank for about 2 years more, until he took it to a lake. We both, maintain excellent filtration and abide by a 30% water change every month, and we fed it vegetables and and blood worms.