:whistling In my teens I once left some roach livebait in a keepnet under some weed. The intention was to use them as livebait the next morning.
When I pulled the net out the fish were covered with leeches
Medicinal leeches were used by Kev Maddocks and others to catch Wels catfish some years back, but as they are about £5-00 a pop (each!) they are definitely not worth it. Top of the pops with the continental catfish brigade are live tench (not cheap to buy) and live silver eels. Rather you than me on the later!
I have caught smaller cats in France and Spain on freshwater muscle, live baited roach and boilies. I think they have quite a large diet, because aswell as a ferocious predator they are a scavenger too. Have heard that halibut pellets does the trick for them too. Another tip is to fish for them with an uptider as it has a lot more power than a 3lb carp rod and even some specialist cat rods.