OP, here’s a video of an Australian guy explaining what happens when fish are stressed:
In a nutshell, they devote all of their energy to their most basic life sustaining functions, leading to a “drain” in capacity for their other bodily functions including the immune system.
Thank you for that! I think I have decided to bring the puffer to the LFS because he will be better off in a more suited tank for him. I know it sounds weird but I get attached to my fish and hate giving him up!