This seems ok, but most LFS sell RO water at less than a dollar a gallon. I would do that, or invest in a high-quality Reverse Osmosis filter. When using RO water, there is no need to treat it with anything, because it is just plain water. This limits risks of the chlorine, and other minerals and heavy metals from getting into the tank water. These things are not good for fish and maybe what is causing your beautiful puffer fish to be sick. Hopefully, the prime has done its job at purifying the water, but I wouldn't take the risk anymore and use RO water. I hope your puffer fish gets better. I am not sure if this is what is causing these symptoms but it definitely could be. My suggestion (though I have never kept a puffer fish), is to keep the water parameters stable, and very closely watch him. The symptoms you listed aren't enough to give a definite solution, or diagnose him, so closely watch him to see if anything else might be happening. I will be thinking about you since I know you love him. Keep me updated.