Thanks for all the advice. I'm still keeping an eye on tank health. Tomorrow (today, actually, since it's past midnight) is the last day of the 5 day Maracyn 2 treatment. My girls are still looking stressed, though not as often, and my alpha female found an old shell to hide in which scared the crap out of me when I couldn't find her.
I'll probably get two new girls tomorrow. By the time they're out of quarantine, I should be reasonably certain that the main tank is healthy. If not, then they'll just have to spend longer in quarantine. Separated, of course.
I'll definitely try rearranging the tank. And I'll do the same floating introduction that I did last time.
Thanks again for the help.