generally for catfishing 10 to 15 lbs line is sufficient. Ive landed some rather large fish on 10 lbs test I once landed a 16 lbs halibut on 10 lbs test. and Ive seen some reel miracles like my buddy rob landed a 33 lbs flathead on 6 lbs test. for 24 to 40 inch catfish id probably go with 12 or 14 lbs line and call it good. alot of us have 20+ lbs line on our cat fish rigs but thats usually because were after big boys in the rivers and are expecting 30+ lbs flatheads.