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you understand wrong, tanks can be used for air, nitrox, O2, trimix whatever you want as long as all of the whips going into it are O2 clean and the valve was rebuilt for O2 use, same with regulators, though the second stages don't actually have to be O2 cleaned quite as rigorously as the first stages since they are not at high pressure. Once the tank is connected to an unclean whip I wouldn't use it for O2 clean requirements, i.e. pure O2, or any partial pressure filling methods due to potential for contamination, but I'd be less concerned with the first stages, to each his own. My O2 regs are dedicated to O2 use, but the rest of them are used for pretty much everything else interchangeably and while I O2 clean them on rebuild with O2 clean parts, I don't ever worry about anything but the deco reg