Is there any decor? They like hiding out in plants & caves. I still think you need substrate & much better filtration. It looks more like a temporary holding tank, then his permanent home.
You have no substrate = little surface area for bacteria to grow (or to let the fish feel it is in a comfortable, natural environment) & very little in way of surface area with the media you are using & no mechanical filtration of any kind.
PP already gave you the run down on what to do for this puffer, so all i am going to say is, that is one nice hairy puffer... looks like an old man already...