Mile - Hi,
I'm using a Sullivan round tank, I think it is 10oz (had to do a little carving to get it in - no problem). I get at least 15 min flights since most of my flying is low speed (per plane design). Only full throttle is vertical climbout. I had to put my battery in the tail to get the balance I wanted - you can see the opening from my underside photo.

The stock tank was fine for flight time with my 72, I went with the Sullivan tank 'cause the supplied tank (World Models) had a crappy stopper on my first SE ARF and it leaked (second year I had it). Sullivan makes a better tank and stopper - no leaks!