Off-flavor is a real big deal for catfish farmers, and lot's of research has been invested to lick it. Their policy is to sample the fish, if it eats well, it goes to market. If not, Sorry Charley.
https://srac.tamu.edu/index.cfm/event/getFactSheet/whichfactsheet/33/
There's just some fish from some places at some times that aren't fit to eat. Anything other than white can be trimmed, which can make a lot of difference. Yellow "shoulders" are fatty and should be trimmed as well. If the yellow is from a gland sac ruptured while cleaning, you can't fix that.
Acidic soaks have been reported to help, but they also dissolve tissue and get mushy.
Cooking's simple. Peanut oil at 380 degrees. Enough fire to keep it there when you drop fish. Some frozen fries to drop the temp if you get over 400. When it floats and the bubbles go from tiny to big it's done. Remove each piece when done, rather than waiting on one piece to dump the basket. Never lift it from the oil until it's done. Thicknesses from 3/4 to 1" cook best. Cut big fillets down if needed.