straight taps need to seal on a gasket or O ring since that can not be done, just goop-ing on some type of sealant or silicone is not the correct way to make that seal.
I would use a valve that had a tapered pipe thread 1/4 to 3/8(depending on tank)on the inlet and drill appropriate hole size and tap with the matching size tapered pipe tap. Tapered pipe taps seal within the threads ,straight taps do not, (thats why the valve had a O-ring on it). you can put a wrap or two of Teflon tape or some pipe dope on the threads,either one just helps seal the thread to seal. just tighten and the seal is complete, no waiting for something to dry.
I forgot "you must" 
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