Sorry, I have no experiences with Puffers, but I keep a lot of discus and aros over the years and they tend to freak out more than most. I find that anything can trigger a fish to freak out and this is their natural flight instinct kicking in. To counter this, I place substantial hiding spots for them and they usually rush into their favourite spot to settle down. Turning off all lights definitely helps. For discus, drift wood crevices work perfectly. I have less success with arowanas due to them being top level fish. I presume your fahaka might benefit to having a 'safe den' it can easily retreat to and like my arowanas, as it grow bigger, it will be more confidant and have less freak outs. I hope this will help, and if you do use driftwood, just try to minimise sharp edges. (never had an incident of my fish injuring itself on driftwood though)