Java loach with sore

[MultipleTankSyndrome]

Chances are if it's a louse, the other fish will get it. With that said, since you only posted the 'eaten away' image today, it's possible the louse only just started the infestation and has yet to get to the others, so keep an eye on them in the next few days.

For lice treatment, you could try Diflubenzuron. Apparently works for lice on goldfish and koi, so could work here if it is lice.

 

[AngryRainbow]

I'll definitely keep an eye out, but the "eaten away" status isn't new today, the loach looked that way almost the next day after my original post. It's starting to heal now.

 

[kno4te]

Could it be some kind of fish louse that took advantage of a secondary infection from the camallanus worms? Those are know to eat away at the flesh of fish and make them listless (see what happened in poorly managed salmon farms).

Should be able to see the louse externally.

 

[MultipleTankSyndrome]

Yeah, I now think if there was a louse it's no longer there. AngryRainbow said it was getting better, which probably wouldn't happen if any lice were present.