Number one. Do u have any other pets in ur house. Number 2 wat do u feed it. Number 3 it will definately not be blowned by the gas as it is in water and if it was blown up it will have jags. Not clean cut. Number 4 other fish attack. Number 5 decoration scratch leading to a opening and while he was swimming snaged it on the decroation or something. Number 6 some fish can be chemically treated to be in a different type of water, as in salt or fresh. Number 7 or some fish can handle freshwater when they are really salt. Number 8 does ur birchir have like a big hole in the bottom of is it a clean cut. Pic would help a lot. Good luck
Sorry for your loss.....does it matter how it happened though? I understand if another fish did it but it may have happened after death and you will never know!
i think the idea of it being ripped open by some decor in the tank and got its insides ripped out on a snag.. or it was the other fish, a pic would really help solve this mystery