Don't use brass or bronze fittings in aluminum on a boat!!
Only use another aluminum fitting, or preferably a stainless one.
You can get a stainless elbow and then thread your brass fitting into that, or get a straight up pipe to hose elbow.
Something like this:
https://www.amazon.com/Brennan-5502-...t%2Belbow&th=1
Or this:
https://www.amazon.com/Stainless-Ste.../dp/B00ZY5IFAI
Of course you'll have to size it for whatever size boss you already have welded into your tank.