On a heavy feed plan of good quality, they should mostly be eating your food and in my (non-expert) opinion would be very UNLIKELY to have elevated levels of mercury. As the catfish rapidly gain weight on the feed, any initial mercury in the muscle mass of the fish should actually tend to a lower concentration with an increase in size.
All of what FishingRod said is true but just wanted to add emphasis. There is no safer fish to eat than fish grown swiftly on formulated feed. The lesser the fish meal ... the lesser the accumulated Mercury also. So fish that efficiently convert feeds with low concentrations of fish meal (like CC and TP) provide the safest fish to eat. These could be consumed daily without exceeding the health thresholds. Noone eats fish this much but as much as you want to eat fish like above ... you should be able to do so without taking on health risk. Indeed, the benefits of consuming fish you grow like that exceed the possible risks.
The longer it takes to grow out your harvested fish, the more mercury they will contain (they will have consumed more food ... both natural and feed). So ... the safest fish will be fish grown while being fed to satiation.
