RE: ARF Fuel Tanks.....
On even more than opjose mentioned, I usually use the included tanks but I also usually only get involved with the better brands.
It depends a lot on the brand - some brands always suply poor quality tanks, others occassionally have a dud included. I have had a Great Planes (Which I consider to be generally a good brand) ARF where the included tank had a pinhole in the seam.
Inspect the tank and see if it has a good seating area for the stopper, if there are any "thin" parts - especially on the corners (usually visible as a clear area ar as a very flexible area) and once it is assembled - check it for leaks.