Before you resort to pliers: Get a length of ducttape; wrap it counterclockwise around the base & tank and leave a nice, big "handle" length; grip the mod and pull that tape like a wrench.
Now, if that is still not enough - leave the tape on and use some channel-locks (or a length of leather) - something between the teeth/jaws and the finish of the atty.
NOTE: if you have bottom airflow, you can also stick a screwdriver or wrench - barstock - into the intakes and use THAT as a wrench as well.
Above has worked well for me, (and I've stuck them in the freezer on occasion as well). It's a really negative aspect of our damned connector-design.