you could keep the channel catfish in that tub year round no need to move him elsewhere. My channel catfish just died recently from a stab wound inflicted by another catfish, but was kept in a similar sized pond, and grew to 28" in just 2 years. That would help you when you move your other fish back inside. What's the coldest it gets in your area during the winter? So long as it doesn't freeze the water he'll be fine outside year round.