Thanks Pat and Kev--I appreciate the information and encouragement! Things had been going so well I had probably gotten a little complacent and had failed to remember how sensitive they can be.
I'm pretty sure my problem has been bacterial, though flukes are also a possibility. It's possible that there was a day before I moved the main filtration to the fish side of the divider that I was having water issues related to ammonia or nitrite due to insufficient flow from one side of the foam to the other. (Otherwise, I do age and aerate my water and the fish haven't been stressed by the WCs.)
I added 1-2 tablespoons/gallon of salt to the tank when I saw the increased respiration and some darkening in case there were nitrite issues and to help with osmotic pressure since they seemed oxygen starved. Have plenty of aeration. Didn't think I was too heavy handed with beefheart flake food and bbs, but in hindsight I'm sure quite a bit ended up in the foam and filter sponge and floss, which I'm now squeezing out a lot more intensively.
Seeing all this, I, too, decided to treat with PP and found that my water was turning brownish very quickly--evidently lots to oxidize despite wipedowns and a generally good appearance. Have done this twice over the past three days and may try (again) cleaning all remaining filter material in tank water and lightly dosing again.
There are a core group of fish that are light in color and energetic, with a few other survivors still in-between. I can definitely appreciate why we started the last challenge in the 10 gallon tank (which I still have in the shed), and if space weren't such an issue I would more seriously consider moving them, though I might then feel as though I had to get 2x/day WCs in, which may be too much for me to keep up with.
Anyway, we'll see if I still have fry to work with--that's the first step--but I'm still hoping things might stabilize today.