^^^ What they said.
Hand tight is best, but if you can't get a good grip on it, then using a basin wrench might be a good solution... and be careful!
Plastic threads, rubber gaskets, and glass tanks are not overly friendly to heavy handed techniques.
I've seen documents that say to put the rubber gasket on the dry side of the bulkhead... I've never had good results that way, I always put the gasket on the wet side.