Depends on where its leaking. If its leaking from the weep hole (if it has one - will be close to the bottom of the adapter) or the bottom of the adapter itself, try changing the o-ring on the tank valve. If its leaking from the shaft of the on-off knob, there should be one or two o-rings on that shaft. Lubricant may help on the adapter shaft o-rings. They can be difficult to remove, depending on the manufacturer.
FWIW, 850 psi direct from the valve is a lot of pressure to run without a regulator. I'd anticipate problems with keeping steady pressure from the needle valve. I have a similar rig on a five gal tank. But I use a paintball gun regulator and adapter/on-off. Even with that, I have to monitor it daily to make sure it doesn't go too far in either direction...
There was a small clear o-ring just under the first lip of the threads on the paintball tank. It was definitely leaking between the black on/off valve and the brass threads on the tank. No soapy water trick needed (it sounded like an air compressor hose).
The funny thing is, the black on/off valve never really shut anything off. Once it is screwed down, it's on.
850 psi vs 1 needle valve? No wonder it was so tricky to get calibrated to 1 bubble/5seconds. Thats a lot of work for one valve even if its not the fail point so far. So my question is, where'd you get the paintball co2 regulator and how much are they? Also, did you hand tighten it onto the tank or use a wrench to secure it?
Thanks for the enlightenment