My buddy that does fab work welds tanks all the time.He runs exhaust in the tank before he welds the.He usually tigs his.I had one friend that was going to weld a tank years ago.Drained it flushed it and let it sit for a week.Went to weld and was one of the loudest booms you ever heard.That tank went probally 50 feet straight up.Thats why me fabricator friend always runs exhaust into them.Rocky