I have seen this on more than one occasion. When there are excess heavy metals, there is this reaction.
Investigate all possible sources of toxic metals; from hose clamps and plumbing to pumps and possibly even source water.
If you deal with a lot of evaporation the unwanted metals are constantly compounding.
Look into Polyfilter by Poly-bio Marine. It is a filter media that strips out toxic and heavy metals. Good stuff.
After a couple of Polyfilters and possibly a couple of water changes with reverse osmosis water, you should notice the puffer's lockjaw disappearing.
Fish do not read books, nor care what we think.