No worries Laurie, its sad but happens to the best of us. We don't learn if we don't try , right?
Unfortunately there is very little to no info available on these cute little critters, the web burrfish puffer. Most species of these are deep water puffers. They are newest to purchasing and becoming more available as demand grows. You would think because they are a puffer and look like a porcupine puffer they would be able to go in a aggressive or FOWLR tank, however thats wrong. They can go with shrimp, however may nibble on snails & crabs though to small to kill them. In short...They are hard fish to keep.
They can't be stressed or they stop eating, and will get picked on by even a simple clown fish. They get super stressed in busy tanks and will hide. They are loner fish. They don't go good with other puffers, as I have experianced, they get bite and can suffer infection easy to their soft tissue (mostly the mouth) and will die. When they are about to die or hours before their spikes go completely white and their eyes go black marbles, losing there blue/green marble color. Then you know they are sick and dying. They are best by them selfs, until they are larger. Which makes them hard to keep. Since they sell them quite small. They even don't do well in pairs and will often kill the bigger one so they don't have to fight for food. The worse is they are the first to show parasites because of their soft tissue, no scales. If they get any disease they are hard to treat. Most products as they slime coat acts up as a defence and don't strive. No copper at all for these guys. These poor guys are parasite and fluke magnets. They can do hyposalinity and tub transfers well. Despite all that, these guys are cool little dudes. So fasinating creatures that will follow you around the tank like a dog.
Sounds like your guy got a air bubble, hence the floating. They puff to tell others to stay away but sometimes they catch an air bubble at the surface and then like sea horses they need a little help to rid it. Most times by simply exposing them to air does the trick to rid. I always ask the LFS to bag my puffy UNDER the water and please not expose to air for this reason. Your guy would easily got it from floating to the corner and sucking in an air bubble. When they puff they have no buoyancy, no control of where they float to, untill they deflate again. Smaller species, can't out swim the powerful powerheads.
Hope this helps.
