When scuba diving recreationaly, Nitrox tanks actually have higher concentrations of oxygen, leaving less “space” for nitrogen and other gases.
Normal air tanks are filled at 21% oxygen. A nitrox tank will have more than 21%. The ones I use have 32% oxygen, wich helps when being at depths between ~20m and ~40m
I know that technical divers do use other gas concentrations, and they even change the nitrogen in the tanks for other inert gasses. But I’ve no idea of which gasses they use.