Okay, thanks for your help. No, there's no sealant between the regulator and the brass reducer. Are you saying that since there's a washer between the tank and the regulator, I don't need it?
Correct. Don't use any type of sealant between cylinder and regulator. Just the nylon washer or permaseal.
NPT threads are tapered. Tapered threads use sealant to create a leak-tight seal. Always use tape, dope, or some other sealant.
CGA threads are parallel. Parallel threads use a washer to create a leak-tight seal. Never use tape, dope, or other sealant.
You *will* have leaks if you don't use sealant on tapered threads. Teflon tape is most popular, and very cheap. Pipe dope works, but do not use it on high-pressure connections (for example, where the CGA nipple screws into the regulator body, or at the connection to the high pressure gauge), as it may literally get blown out of the threads.