Minecraft wiki entry for Pufferfish, associated text: A pufferfish inflates when approached by a player in Survival or Adventure mode, a drowned, an axolotl, or any non-water mob within a 5×5×5 volume around the fish, going from unpuffed, then semi-puffed, to fully puffed. Pufferfish are technically passive mobs but going near one when semi-puffed or fully puffed inflicts the player/mob with three or six seconds of Poison based on the inflation level, and touching one in its puffed or semi-puffed form deals damage separate from the poison.