Mercury also tends to concentrate UP the food chain, especially in top predators that eat forage that ate smaller forage that ate even smaller forage that had consumed some mercury.
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.
If you are going to eat LOTS of catfish from your pond for many years, maybe you could test a few of the largest ones for mercury. No idea where to get that test, but if you are close enough to the coast, one of your nearby universities might be able to perform that test. Or perhaps a state agency.
P.S. That follows on cb100's comment about local mercury levels. If they are very low, then I would definitely say "no worries".
