It was my fault. The fishes looking poorly in the window tank. This morning one cory seems to have small white marks on its fins- fuzzy- and Fabio definitely has fungus on his torn fin, and the fry's gills look red. Since the day I dumped a trumpet snail out of the filter box, it's had a dull rattle- not very noticeable so I did nothing. And flow seemed slower- I thought it was from the media components, but they weren't clogged. And there's been more small particles in the water than I ever saw, and that dusty film on surface.... water was looking cloudy. Now I'm surprised I hadn't killed all the fish with this neglect already.
Because just now I emptied the filter box again during a water change to try and help the fishes- and found that the impeller was loose. I tried to fit it snug again and increase in flow significant- and all the fish immediately swimming around active- Fabio doesn't hang out in front of the filter output anymore and I feel so bad I hadn't caught what the problem actually was, earlier. All the cories are exploring around the tank without those frequent quick dashes to the surface I'd been seeing so much lately. I do believe they were oxygen deprived- I had checked the water for ammonia but didn't think about oxygen or flow. Hopeful to see if the film dissipates (it's gone now) and the fish health improves.