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Thanks for moving this thread.
Water parameters would be helpful - especially nitrates. There is a large bioload in this 125 gallon!
Have you fed the puffer anything besides the krill? Do NOT give anymore krill. Try adding Vitamin B12 to the tank or vitamins with B in them. Try feeding squid, shrimp or crabs from an Asian market
Unfortunately this is a "common" problem with predatory fish especially puffers and lionfish that are fed a diet soley of krill. This diet leads to dietary deficiencies and a condition called "lock jaw". While the puffer appears to be thriving - nutritional/mineral deficiencies are insidious and take months to develop. Once they start or symptoms (not eating) occur, reversal is not always possible.
Overgrown teeth are a possibility for going off feeds or inability to eat - but with the diodon puffers, overgrown teeth is not problem. The dental plates are two fused plates. Arothron puffers (dogface) have 4 fused plates which form the classic "beak" which lends to overgrowth problems.
How long have you had this puffer?