On that same note...I went to Sports Chalet last night to get an air fill. Long story short, there was a Sports Chalet instructor there who refused to fill my cylinders because they had a "nitrox" sticker on them (they weren't even "enriched air nitrox" stickers). He proceeded to tell me that adding air to the nitrox would poison me...kept saying it was poisonous. NCIS, alright, I can kind of forgive...but a scuba instructor?!! And by definition, isn't air nitrox?
I'd go back in with a "21%" test marked on it and see what he says.
I'd say to add a MOD but as we all know "how deep is too deep on air" is a whole 'nuther can of worms.
PS- and JAX - thank you for changing your avatar. Hard to see a GW pic for an HD rider like yourself