A 3000 gal could house several large catfish with proper filtration. I'm still gathering materials and planning my big tank/pond(some argue it's both), but in the end it will be around 4500-5000 gal and will house about 20 monster catfish. The filter will be in the 500 gal range. So dream big. 3000 gals is much larger than you think. I currently have a pool that is 1100 gal with 8 catfish that can all exceed the 3' mark, but for now they range 14-18". BTW I hate my pool. everytime I bump it or do a water change or move a rock or filter it starts to leak. Large cattle trough, or home made pond with actual pond liner is a much better idea. I'll be building a timber pond to rehouse my fish until their mega tank is built. I can't stand the thought of that leaky pool in my garage any longer.