Sixlines are the worst fish I've ever dealt with. He'll probably do this to more fish in your tank sooner or later. If you can get him out (which you probably can't) I'd suggest you do. I had a sixline kill many fish on me before I finally was luck enough to get him stuck in my overflow. Brought him as a "gift" to the LFS, the owner told me that within 10 days the fish had killed numerous fish in each tank that he tried to put it with and ended up giving it away as a freebie to someone who likes aggressive fish. (I think he just flushed it but didn't want to tell me that). They're one of the most aggressive fish I've seen, especially for their size, he definitely chased your other one out of the tank.