Need a lake imo. One of the largest fw fish in the world. Something in the 100000 gal plus range would do fine. footprint of at least 40 x 80 foot wit at least 8' depth.
I am also under the impression that wels is restricted in the USA unless things have changed. I remember reading the law said it and all of its hybrids were restricted.
Agree with Necrocanis. A hole in the ground is the cheapest. Excavation rates vary depending on machine, usually from $100 to $300 an hour. With soft soil, looking at a few grand, i'd say.