You will need either these, or some inline ones. These should duo the trick as it is what I use.
https://www.amazon.com/gp/product/B08FQ3W79Z/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1 or NPT
https://www.amazon.com/gp/product/B08FMWGP8D/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1 I set mine up differently than most people would consider. I removed the Blow off valves for my SCBA tanks, replaced them with the Male Foster fittings with the built in check valve. I also removed the fill whips gauges and replaced them with the male foster fittings with check valves. I fill my tanks by daisy chaining them together off one compressor currently. In the market for a Pro fill station....
Here is how I fill mine. - Tank one, gets the Compressor connected directly to the foster fitting on the side of the SCBA tank. I put a fill whip in the output connector of that tank, connect that one to the next tank to the foster fitting on the side and keep doing the same setup for all 3 or more tanks. I open up all the valves of the tanks with fill whips and they cascade fill each other. Filtration and moisture traps are used as well as bleeding off the compressor etc.. Plus once I get up to pressure, I take the tanks, Invert them bottle valve down. Let them cool, then give a couple quick valve cracks to remove any moisture. Check pressure, top off once they are all ambient temp.
What you could do to add something in here to help with the filling. Wether it be you have multiple tanks and daisy chain them together. fill at both ends. Or get a High pressure rated 3 way fitting. 2 male foster with check valve. one with it removed, or 2 male foster with check valve and one female foster coupler depending on your fittings you use to fill.
When I fill PCP's. I hook each fill whip up to the next fill whip with the male foster fittings. Then I Cascade fill. Crack the lowest pressure tanks valve. Fill the gun to that pressure. Close the valve, onto the next tank that has the next highest pressure, crack valve, fill, repeat. I use 3 tanks, so I do this 3 times. I also use a pressure release valve and check valve at the end to loose as little pressure as possible. That way, the system can stay pressurized all except the last couple inches off the fill whip at the end. Perfect? No. Saves me some time and air in the long run. Yep. I normally end up filling most peoples PCP's when we go shooting. My system allows for others to Connect their tanks in tandem with the rest of the system so we never run out of air. Still;l always start with the lowest pressure tank as first in the system. I could just make a manifold with valves to turn with all the tanks attached to. Cost is a factor.
Follow me here before all of you start biting my head off about safety, etc. The Blow off disks are rated to 3XX Bar... They hydro test the cylinders at exponentially more than that. The Male Foster fittings with the check valves are rated up to ~5k max fill. Yong Heng and similar aren't rated up to 5k. If you keep an eye on things like you are supposed to. You will never overfill or get in the danger zones of the tanks.
