I'm stoked to have come across this thread. When I got my tanks I tried flanged and was bummed because my brain said 'no recess = no flange' and move on... Today when I went to refill and did a big spill I got to researching again... Now, ill use flanged cuz it just works for me... I appreciate everyones help-so much!!! and very glad I hadn't processed my carto order yet!!!