What should recreational divers never use to fill their cylinders?

Recreational divers should never use pure oxygen to fill their cylinders because it significantly increases the risk of fire and combustion. Oxygen is a highly reactive element, and when it is pressurized in a cylinder, it can create a very dangerous environment if there are any flammable materials or sources of ignition present.

In recreational diving, the breathing gas mixtures like air and nitrox (which is a mixture of oxygen and nitrogen) are safe for use within the acceptable depth limits and provided that divers have the proper training and equipment. Helium is also used in specific deep diving situations but not in standard recreational diving. Therefore, the only gas that poses a significant hazard in this context is pure oxygen, making it unsafe and inappropriate for use in filling cylinders for recreational diving.