needle valve
i'm not 100% sure but i believe the needle valve is inside the regulator.
you screw the regulator on top of the co2 tank, it usually has two guages. One for how much co2 is in the tank and another for the pressure going out.
the needle valve is just regulated by a screw, tighten it for less pressure, loosen it for more. you'd have to make sure the regulator is for lower pressures so your not shooting a ton of co2 into your tank.
then you just run a line from the regulator to your co2 reactor, or simply into the input tube of your filter.
i could be totally wrong but i've worked with welding and torching tanks and you regulate how much gas going out by turning the screw, not difficult at all.