All copper is toxic to pufferfish. You are killing him. Let me go see what they usually do for them. All he needs to be in a tank by himself and you need to try to feed him clam on half shell , shrimp. This has to be ich not velvet as they all would be dead by the end of the day. Our puffer got one case of ich. I bottle a bottle of garlic and soaked her food in it for a week. Never has gotten ich again. You have to boost their foods with selcon and vitamins.
Where did you read that copper is toxic to them. I have never seen any issue with copper and puffers, particularly in retail copper systems.