Use the proper (new) O ring, make certain that it is properly positioned and the mating surfaces are free of debris and do not overtighten. No goop.
It will leak if you do not use the proper O ring and/or the O ring grooves are not clean.
It will leak if the O ring is not properly positioned.
If you over tighten the bolts, you will damage the valve and/or tank and it will leak.