I agree that the aftermarket tanks should have identical outlet connections (1/8" FNPT) which are pretty easy to get sealed. You're saying that the tank has a male flare outlet?
I'd be sure where the actual leak is (if it's at the tank fitting from the supplier, you're pretty much hosed, although maybe a crack could be soldered closed, which can be dangerous without safeing the tank). Copper should use a single flare as Dave said, and should seal up pretty well on a standard flare fitting. It's used for stuff like fuel oil, etc.
I'd install a short stub of copper and transition to a compression style fitting (with nut and ferrule) below the floor to work similar to the original pipe nipple. Or you could even sweat a male NPT fitting on the lower end of the copper for the original style shutoff valve.
on edit: if you're having trouble with getting a flare fitting sealed up, a little bit of pipe dope on the flare helps.
Last edited by klhansen; 08/14/2018 12:03 AM. Reason: fixed compression style fitting description
