i feed mine market shrimp, cod fillet and beef heart, they eat it up like they never ate before, i threw in some hikari pellets just to see what happens and they ate it up to, give it some time if not maybe its your water parameters
What kind of pellets are you trying and what kind of bichirs? My big ornate doesn't get pellets because they're too small (and expensive!) but my little dels love hikari sinking carnivore pellets, cut into 6ths!!