I got my first Aerotank today and had the same problem, because I inadvertently torqued it up (the airflow adjustment wheel) while struggling to un-tighten the
threads for my first filling. I will say that I eventually had luck with taking the whole bottom piece off (where you fill it), dried it off so I could get a grip on the area just below the "knurled" airflow adjuster, and just tried like the dickens to grip it with one hand and untorqued the adjuster with the other. It was frustrating at first but keep at it! Mine may have actually loosened up due to some liquid getting into the
threads and acting as a lubricant. It certainly helps my iClear XI when the
threads feel a little stiff.
Edit: apologies if this is a typical recommendation. If this hadn't have worked, I would have taken needle-nosed pliers or perhaps a smaller pipe wrench, wrapped the gripping areas in electrical tape, and used that (I don't have a decent vice or clamp at the moment) to grip the smooth area that the adjuster is budged up against, while still trying to just use finger strength to un-torque the adjuster. That might be a typical one too, but I think the key part is just making sure you can isolate a tight grip on the smooth area and a separate grip on the adjuster and to be sure you're turning the right direction...a few minutes into it I looked briefly at the instructions and did a "righty-tighty" exercise in my brain just to be sure I wasn't going crazy and going the wrong way.
