Many years ago, a buddy of mine bought a small channel catfish for him 90 gallon. It grew very quickly and became a chunky resident quickly. It was a good fish for him topping out between 12"-18" a year later. It died about two years old in a power outage one hot summer. I would not advise a 90 gallon (get bigger) but you sure don't need an outdoor pool. Channel catfish can get quite large and large tank is the most natural for behavior and a well shaped fish.