Welcome to MFK! The fahaka puffer should be OK if it has at least 18" width. One thing to worry about is that these are messy eaters, and need pristine conditions. Personally I wouldn't house them in smaller than a 125.
I recon 100 gallons should be fine, if the width is of a good width (sounds daft), and you have really efficient filtration.
They are a great puffer species!
Welcome to MFK! The fahaka puffer should be OK if it has at least 18" width. One thing to worry about is that these are messy eaters, and need pristine conditions. Personally I wouldn't house them in smaller than a 125.