The fish (and several other 6-month-old spawnsibs) was in a ~20 gallon plastic storage tub with a HOB filter. Now the affected fish is in a smaller (7.5 gallon, I think) tub with no filter, because all my filters are already in use. I discovered the condition while doing a water change. I do about 25% water changes twice a week on the ones in the basement where it's cold and the fish aren't eating or pooping as much (and daily or in a crunch every other day on the ones in the living room where it's warmer). Water parameters are generally good when I test them, but I didn't test this one today because I was in a hurry so just did the water changes and discovered the sick fish and got it out into a quarantine tub with all clean water. I then added dissolved salt, 1 tablespoon per 5 gallons, to the quarantine tub and to both of the larger basement tubs with baby fish in them.
I didn't get a picture either. Will try to get one when I get back home this evening.
But meanwhile while I'm out, if I can get to the LFS before it closes, what should I look for?