Dude ... the rubber 'gromet' or stopper is not enough to seal the tank properly. The metal discs are used to sandwich the rubber stopper, the rubber will expand as you tighten the screw (sandwiched effect - imagine this with 2 nice lanky gals ... hehehe ... ) and this will seal the tank properly. Please do not over tighten the screw as you may pop the tank. Just see that the rubber stopper is slightly expanded and that it is fitting tight, firm and snug.