[Jacob]

Thanks for the information. Is there a fungicide ingredient that is best?
It looks like the patch is slightly larger and more cotton like but it also looks like it is hanging by a thread. The sick fish is still eating, and I noticed that it and its mate still are showing the occasional strange shuddering. Maybe that’s “shimmying” and is a clue.
But that symptom isn’t enough to justify trying to medicate the whole tank, it seems.
I will try and take a picture but I only have a cell phone and an old camera that may not be much better than that. Maybe it’s time to get a decent camera.