Hi all!

New to the forum because I'm trying to tackle my first plumbing job on our house. I'm installing a Culligan water filter but can't seem to stop the connector joint from leaking.

I'm such a newbie that I don't even know the terminology, so bear with me:

I have a 1/2" tube going from my water supply to a brass T-joint that has a 1/2" opening and a 3/8" opening. From the 1/2" opening, a tube runs to my faucet. From the 3/8" opening, I have a 3/8" to 1/4" adapter-thing, where the tube connects and runs to the water filter.

When I turn my water back on, a very small leak comes from the 3/8" to 1/4" adapter and I can't seem to get it to stop. I used teflon tape and made sure the nut is on tight, but still can't figure it out. (See photo, the red lines indicate the leak.)

Any ideas? I'd love to figure it out myself without having to call a plumber, because I feel like I'm so close to being done!

Thank you in advanced!