The feeder fish is a cyprinid. Perhaps Cyprinus carpio carpio (common carp) or
Carassius carassius (Crucian carp or normal colored goldfish ancestor).
I am surprised you have not given cut fish a try. 100x higher success probability than pellets IME.
Ok... I’m surprised I never think of that too. But at this moment, I will try my best to breed tilapia for them. When successful I will breed African catfish as well.