You shouldn't use teflon tape if it is a plastic fitting. I don't recall why, but every plumber I know has said this. The only time you want to use teflon tape is when it's metal to metal. Teflon tape was invented to not be as messy as before when there was only "pipe dope" which is still available today.
I recommend you remove the plug and check the threads, it they look good you can apply pipe dope on plastic to metal fittings. Make sure the pipe dope is applied completely and evenly but not too thick and not too close to the inside edges of the threads (you don't really want this stuff in the tank). Or you can use food-grade pipe dope and not worry about it.
I hope this solve your leak. I hate plumbing. It always seem to leak no matter what, but keep tightening a little at a time until the leak stops.
I recommend you remove the plug and check the threads, it they look good you can apply pipe dope on plastic to metal fittings. Make sure the pipe dope is applied completely and evenly but not too thick and not too close to the inside edges of the threads (you don't really want this stuff in the tank). Or you can use food-grade pipe dope and not worry about it.
I hope this solve your leak. I hate plumbing. It always seem to leak no matter what, but keep tightening a little at a time until the leak stops.