That would most likely be a 1” bulkhead. The flange and gasket goes inside the tank with the nut on the outside. No pun intended.
If you have some silicone grease, I like to use a dab on the nut flange and threads. Not totally necessary, just tightens better. Do not use a wrench while tightening. As tight as you can get with fingers is tight enough. Using a mechanical device (wrench), you are most likely going to crack the bulkhead and it usually shows up as a drip leak AFTER you have it all set up. I witnessed it with a friend’s 220 gal with a 6” DSB. Not a pretty site to replace.
Dick
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