Sorry I have not been on this forum in 6 months. I repaired tank by using the 3/4 FPT spin type fitting that was used to originally built tank. They are available on line & cost about a buck plus six bucks or freight. I plastic welded it in place using a combination of a heat gun, a harbor freight plastic welding tool and s/s mesh, and plastic rod that was included with welding tool. I used it last season and seems fine

Have you had any luck with your freshwater tank? we are in the same boat. The drain cap was busted off and we are having a heck of a time finding a replacement.