As with other monster fishes, you want to give them a variety of food as their diet, with pellets being the main course. Flakes are not very nutritional IMO. Stick with pellets, I highly recommend Hikari Sinking Carnivore pellets. Occasional treat such as krill, bloodworms, mealworms, market prawn would be appreciated.
feed mostly invertebrates like insects or freshwater shrimp.
Dont overfeed them, for bleheri its pretty normal being not very active.
This species is subtropical, so do not keep them too warm.