Yes, nitrogen. Your compressed air is already 78% nitrogen so another 22% is not that big of a stretch. I have been using compressed air filled by my local welding supply shop but compressed air is a pain. First, the shop has to have a compressor to fill the tanks which in my case are 444cuft capacity at 4500psi. The tanks need to be hydrostaticaly tested, and my were when I bought them and the tanks are stamped with the certification as well as on the paperwork. Next you will need the proper valving for the tanks, which for compressed service is the cga347 valve. The output of the valve can the be adapted for your fill station. But I would not advise just a Jill station setup since that does not have a regulator. I, instead have a Harris regulator good for 5500psi input and I can dial up any pressure I want on the output. So in my case I set my regulator for 250bar and I can feed directly into my gun or a smalle pony tank. However, the disadvantage of compressed air is the filling shop has to have a compressor. In an area like central ohio, it's hard to find a reliable dive shop or even a welding shop, mine ships my tanks to another welding shop that has a new compressor, that you can depend to fill a tank as big as mine. Another disadvantage of compressed air is it loves to hydrate itself with moisture which is a constant problem. Because of the problems with compressed air I will be switching over to nitrogen, which can be filled at my local supplier w/o shipping my cylinders to another shop. So how does the welding shop accomplish the nitrogen fill at the shop is that these facilities have liquid nitrogen in bulk storage tanks and filling is very easy without the need for compressors and nitrogen is more inert than air and does not attract moisture, it is completely dry. All I have to do is put different valves on my tanks and a different adapter for my regulator. Now how long can I go before I need to refill my tanks. Well u do the conversion between 444cuft and a 580cc airgun tank, you'll be blown away. Why Iam a presently going this route, just look at all the compressor videos out there, what a hassle they are. Fill for my tank-$40 per tank.