My Red Tail Catfish's diet.?

Currently my 3 inches RTC housing a 75 gallon aquarium. My RTC currently eats live fish as it's diet everyday. But when I get some TETRA MONSTER PELLETS for my RTC, it literally does not have any interest on it at all! I'm starting to concern bout my RTC as he keeps on eating small fishes but not taking highly nutritious pellets. Any suggestions you would recommend? Thanks