A scuba tank for recreational diving should be filled with:

A scuba tank for recreational diving should indeed be filled with pure, filtered compressed air or nitrox. Pure, filtered compressed air is the most common gas mixture used in recreational diving, as it is breathable and contains approximately 21% oxygen and 79% nitrogen, which is safe for most diving activities at recreational depths.

Nitrox is a mixture of oxygen and nitrogen, with a higher proportion of oxygen (usually between 32% and 36% oxygen), which can provide benefits such as extended bottom times and reduced nitrogen absorption. These mixtures help mitigate fatigue by lowering the amount of nitrogen that enters the diver's body, reducing the risk of decompression sickness.

While options referring to oxygen only or a helium mixture have specific applications in certain types of diving, such as technical diving or deep diving, they are not suitable for general recreational diving. Oxygen only is dangerous due to oxygen toxicity at depth, and helium mixtures are used primarily in deep or specialized dives to counteract narcosis but are not commonly required for recreational diving. Room air, although breathable, is not as filtered or regulated as the compressed air supplied for diving, which is essential for safety and performance underwater.