How do they refill scuba air tanks?

You have to refill them on the dock or the boat. You can’t refill them under water. Dives are considered a one tank or two tank dive. I don’t know what percent oxygen is used; I always assumed oxygen is just oxygen but I’m nto sure.

They are filed with atmospheric air (20% Oxygen and about 80% Nitrogen) for normal dives, there are exotic mixtures with Argon or Helium instead of the Nitrogen. Pressure is normally 3000 PSI in tank.

SCUBA Compressors I use to hang-out at dive shop, they used a water tank to put tanks in to keep them cool.

Actually, tanks have what we call air in them. Air has oxygen, of course, but you wouldn’t want to breathe pure oxygen from a tank.

Also Nitrox. “Nitrox refers to any gas mixture composed (excepting trace gases) of nitrogen and oxygen. This includes atmospheric air, which is approximately 78% nitrogen, 21% oxygen, and 1% other gases, primarily argon.In the usual application, underwater diving, nitrox is normally distinguished from air and handled differently.The most common use of nitrox mixtures containing oxygen in higher proportions than atmospheric air is in scuba diving, where the reduced partial pressure of nitrogen is advantageous in reducing nitrogen uptake in the body’s tissues, thereby extending the practicable underwater dive time by reducing the decompression requirement, or reducing the risk of decompression sickness (also known as the bends).”

From Wiki…

Most tanks are filled using compressors that pump atmospheric air into the tanks. If they need more O2 in the mix, they use supplemental tanks of O2 or both have N2 and O2 tanks to simplify the mixing: https://en.wikipedia.org/wiki/Gas_blending_for_scuba_diving