Fahakas are pretty quick. Normally very active. When mine was small he would keep chasing the feeder until it was tired and then eat it.
Truthfully I have never really seen a fahaka that wasnt always swimming about, especially at that size. What temp is your tank at?
The ultimate is when they get a rosy and eat everything but the head, which slowly floats to the bottom. Its so amesome! My gf hates it, but thats why they are so bad A.
My puffers eat worms, frozen krill, brine shrimp, clams on the half, crawdads, and ghost shrimp. The fahaka eats everything that moves inside the tank.
I have a 10 gallon set aside for feeders. When I get feeders I normally but them in there for a while and feed them heavy on veggies and other healthy stuff.
This lets me know there are no disease, and also gets them full of the nutrients I like my puffer (and others) to get that they dont freely eat.
Either way its only a matter of time before he becomes the JAWS of your collection.