Sudden Red Mouth Weather Loach

[Laanan]

I've noticed that during water changes, and sometimes feeding, one of my weather loaches gets a red mouth (see attached photo). She has been lethargic lately, while the other loach zips around too fast for me to even get a good photo. Is this a fungal infection? I purchased tome erythremycin from the fish store, but I did not want to do the treatment unless absolutely necessary. If I do the treatment, does it mess with the good bacteria, too?

[Diana]

Red mouth might be bacterial, probably not fungal. But it would show up at other times, not just turn red when you do a water change.

Erythromycin is an antibiotic that targets Gram positive bacteria best. There is some activity against other bacteria.

Most fish diseases are caused by Gram negative bacteria.

Often people will combine treatments with something active against Gram negative bacteria and Gram positive bacteria, because when a fish is sick you have no way of telling what is causing it, and there may be 2 (or more) bacteria attacking the fish. Once it is weakened by anything and one bacteria moves in, another often follows. This secondary bacteria may be one of the Gram positive types.

The primary disease is almost never fungal, but once there is some damaged tissue fungi can take over.

Most of the medications designed for aquariums have little impact on the nitrogen cycle bacteria. Part of the reason is that these bacteria live in a complex film on all the surfaces in the tank and most fish bacteria are too weak to get into this film in large enough dose to affect the bacteria.

[Laanan]

Thank you so much for the comprehensive answer. I really appreciate it!