[castaway]

No I have to disagree
It sounds like this puffer is in and advanced stage of sickness formaldhyde will not kill him. In an advanced stage the disease will kill him first.
Formaldhyde or Formalyn unlike copper doesn't linger in the water it is the main ingredient in Quick Cure made by Aquarium Products.
I've used it on fish that are very near death.
I've been in this hobby for a very long time and have tried most if not all medications out there. I've spent money and been sent samples of all kinds of "miracle"and "natural" cures only to have lost the fish and money spent on the product.
If you can find a product called "Dip Away" by aquatronics it is a very good product to dip the fish but it is very harsh and most stores don't carry it.