I've not verified it but I have been told that the main difference is the way the refilled tanks are handled. Many of the welding gases are shipped on open trucks and that lets more junk get on and in the tank valve. Peps, Coke and beer trucks are normally enclosed and the gas may be considered more "clean". I read specs like 99.5 % pure versus 99%? Fish will not care so go for what works best to refill.