To unscrew that fitting with the inside knobs or as you said "tabs", try using a "spud wrench" which is a tool used by HVAC people to replace union fittings on cast iron radiators. Your local HVAC company may have one you could borrow for a few minutes.
Well, I have found my socket set had a wide blade that fitted some, but even some large allen keys can fit.
Or cut some flat bar stock and bend for a handle. Lots of options really as it does not need to be stupidly tight, especially as some ptfe would be appropriate.