1st of all, you are going to need to keep a very close eye on your water parameters. Since your tank will be cycling, I would test your ammomia & nitrites several time a day. As soon as you see a rise in either test, do a 20% water change. If it's really high, (over .5) do a 50% water change. I would use Ammolock or something comparable as a dechlorinator. Make sure you have a good water flow & add a airstone, because ammonia cuts down on oxygen.
Get some Melafix, it works great with bacterial infections & is a wonderful all around tonic. I can be used in conjunction w/other meds & is all natural.
What kind of foods are you offering? Fish are not usually on the menu for puffers. They eat any kind of shellfish, worm or mussell. Your best bet right now is feeding it blackworms in a cone feeder.
Do you have any fish in the 55? The fahaka can't go with other fish.