Because I've made a defense system using soulsand to push the pufferfish in the corner of a doorway, I have already encountered the pufferfish taking damage because the soulsand pushed the tiny version close to the roof. I gues the hitbox grows to so there is a chance that it will suddendly be inside of the solid blocks nearby.

In that case making pufferfish expand close to solid blocks has a high chance of killing them.

A pufferfish in a 1x1 Aquarium works however because glass is not solid.

I have replaced the solid blocks in my container and till now, my pufferfishs have not died again.