When they were living in their small vase, I used to separate them when I would clean out the tank, and then add them back in seperately, before I knew which one was the aggressor (right after I got them) and after the fighting stopped, when I would seperate them, they would get really anxious, wouldn't stop swimming around in circles, would even ram into the temp containers until I put them back together. They seem to do a lot better when they are together than when they are apart. I also noticed that when I stopped seperating them when cleaning, it left the hierarchy in place and led to a much more harmonious bowl. I only add a piece or two in with salt just to make sure that any harmful chlorine is neutralized. (I'm a nursing student and know a lot about salts, electrolytes, and how to balance them properly). Do you have any idea what could be causing her sudden change? I would think it was simply stress, but after looking at her a little more closely, I've noticed that her gills seem to be swollen and pale on the edges. The other one looks fine.
Also, I've wanted to add more fish to this tank, especially now that it is big enough. I just don't want to throw off their "rules" and have them lash out on new mates. Any tips for this as well? Maybe slowly adding three more in? I have plenty of room for four or five of them.