Pufferfish inexplicably dies

I made a 3x3x3 fish tank which was supposed to contain a single pufferfish and placed seagrass and sea pickles on the tank's floor.

I released my pufferfish in the tank with a bucket, and got back to wandering around the tank to build the rest of the room. But after a while my fish died for no reason, I saw it dying and disappearing in a white cloud IN the water and the wiki said nothing about pufferfishes dying in certains conditions, so I don't know why the fish died and I think it's a bug.