With unions on plastic storage tanks or anything non breakable i just tighten them real tight(like until rubber bulges on gasket and plastic looks all distorted). On glass i just go hand tight then a couple wrench turns. Generally if you have a deceint quality union it should be pretty well sealed at slightly past what you can do with your big strong hands.
or you could just order a can of flex seal from the infomercial and coat everything. Did you know that they put a screen door on the bottom of a boat and flex seal made it float
