There is a similar thread on loaches.com.
https://forums.loaches.com/viewtopic.php?f=1&t=48124 Not exactly what you've seen, Claire.
The upshot: she contacted a local vet that treats fish. He said fish can often heal themselves, her addition of salt was "good", & to watch it to see if it get worse. He "can" look at it under a microscope, but for now to do nothing. If it looks worse in 10 days or so, then an OTC gram negative antibiotic should (might?) work.
the loach, I'm not a big fan of "natural treatments" or "immune boosters". If I think my fish needs an antibiotic, I use a fish safe OTC chemical 1. I rarely treat using them. I imagine it's quite expensive to have a bacterial culture (etc.) done, plus the cost of a drug. So I ask, how much are you (or anyone) willing to spend to
possibly help a fish of unknown age when it doesn't seem be long term adversely affected?
I'm really not trying to pick a fight, but I'm a pragmatist, my husband is a medicinal chemist & we're "science people". & to me, there is a cost/ possible benefit element to fish ( or other animal) treatments. This it not the same for everyone, I understand that...