Just wanted to add that PURE nitrogen, argon, or helium are the best (most available) choices. Neon or xenon would work but they're expensive and less available.
Do NOT use CO2 or a CO2 blend, as mentioned it causes the suffocation panic response as blood levels of CO2 are how the brain tells...
Please don't use a CO2 gas blend or straight CO2.
Blood level of CO2 is what induces the suffocation panic response. Without CO2, an animal (or human) will simply lose consciousness with no distress.