Mobile Bay Reef Keepers

I am looking for some help to stop this leak. I dumped an entire bottle of co2 this weekend and thought i had a bad seal to the co2 tank. Got tank filled today and almost lost entire bottle while i was out of the house. Just dumped a bunch of soapy water all over the regulator and found a leak where the high pressure guage screws into the regulator body.
What can i use to seal the threads?
Thank you!

How is yours constructed? Some recommend teflon tape, but I don't. Bits of it can get caught in the regulator and cause it to fail. Check the gauge and see if it has a gasket/washer or something similar. It may need to be replaced if it uses one.