What would be the correct way to thread a brass bar fitting into a plastic end cap? I'm working on building a home aquaponics system with the water coming in and exiting through a similar end cap. Is there a proper sealent that should be used. The end cap is 3" PVC, and the hose barb is a 3/8" brass NPT fitting.
I'm assuming US standards based on PA being listed in your profile.
Plumbing threads are commonly NPT - National Pipe Thread. It's also called IPS, or Iron Pipe Standard/Size. You will also see FIP and MIP for female iron pipe and male iron pipe.
Barb fittings are pretty easy to find with male NPT thread (aka MIP) on one side, made of brass. Here is an example from Granger but you can find them elsewhere.
You always want to screw metal into metal - due to differing thermal expansion and contraction rates you can wind up with a cracked fitting if you screw metal into plastic. So if you're using PVC, you would use a PCV to metal adapter like this - glue it on like any other PCV fitting. Or with PEX, you would just crimp on something like this.
To attach your male threaded hose barb, wrap with Teflon tape or coat the threads with pipe dope, and just screw it in to your newly installed FIP fitting.