A decent quality commercial catfish feed will grow catfish faster than feeding them minnows. As for pond shape, exactly as ewest stated. Minnows are good forage for small fish only. Once fish get bigger, the fish burn far too many calories chasing tiny meals, so weight gain slows until it stops completely. It would be like you trying to gain weight, by eating one raisin at a time, and walking 50 feet between each raisin......you will burn more calories than you can consume each meal...