The reason catfish (and other) pellets float is so you can see if you are overfeeding your fish. If the pellets sink you can't tell if there is food remaining, especially in a pond / lake / murky water environment. Also, any uneaten food is more easily removed if it floats. It's a fish farming management thing. No other reason. Smaller pellets for fingerling and juvenile fish usually are slow sinking. This helps with the transition to train the bottom feeders to look up and come to the surface.