Thank you,The gills look bad. I've never heard or experienced gill damage by tank mates, unless by a candiru or other parasitic fish that suck blood similarly.
Since other fish have been and are ok, this crudely rules out ammonia damage, etc., so I tend to think this is a bacterial or parasitic illness and requires serious troubleshooting to arrive at a guess where it might have come from, or maybe was it latent or dormant and if it was, why it awakened, why the immune system that had kept it in check now failed, etc.
Of course, if this is a fluke, a rare occasion, you might never know.
If really motivated, you can take take it to a pathology lab or look at swabs and scrapes under a microscope yourself, if you know what you'll be looking at.
yes I am a bit perplexed and very worried. The other fish seem fine but I am very concerned something could happen.
I do not own any microscope equipment and am unsure how to prepare/send to a pathology lab, but am considering looking into it when I’m home from work this evening.