I went through the same thing with mine. They killed many fish very quickly. They are beautiful fish though. But if you don't want to lose any tankmates, it may be wise to go ahead and sell her. It's too bad that you don't have a spare tank for her. Sometimes it's a lot easier to get rid of large Festae than it is to get them again. I sometimes regret selling my pair. But at the time, I just couldn't deal with the mass killings and I only had one large tank. I'm sure I will pick them up again when the opportunity comes back around.