For those keeping score at home, the ER thread is over here: http://forum.simplydiscus.com/showthread.php?133466
As mentioned above, the symptoms seen above turn out, it seems, to have been nitrite-related. (Learned something there, that's for sure. Maybe this will help somebody else cycling on the fly, or going through a mini-cycle if they lose their filter bacteria.) I'm waiting for my methylene blue to show up tomorrow, but in the meantime have been using 4x the normal dose of Safe and some kosher salt with my daily water change, and the fish are doing better than they looked above. One of the smaller ones still darkens and hangs out with less appetite than the others, but I'm hoping the methylene blue, some time, and some clean water should help him get back to being himself. Didn't test the water today, but as of yesterday I was finally seeing some significant nitrates, so hopeful that the tank may finish cycling quickly. Crisis averted! :sun: