If your fish are suffering from water quality issues it can lower their resistance to infection. Their immune systems really take a hit when they are having to deal with toxic byproducts in their water.
What you are calling "mouth rot" may actually be columnaris, in fact that very likely is what is wrong with your fish. Columnaris is a very serious infection that needs treatment right away with broad spectrum antibiotics.
If you can post pics of the fish, as close up and clear as possible, it would be helpful.
Until you can, please describe exactly what the areas look like.
I don't remember if in your post you mention that you have quarantined the sick fish, but this would be very important to prevent the spread of illness to others in the tank.
Also, pristine water is very important to help them fight off whatever this is, so do big, frequent water changes to get those nitrates to safe levels.
We're pulling for you and wishing you success in helping your fish get well. Keep us posted.
Please, provide close up pictures and/or a very detailed desription of the lesions.
What you are calling "mouth rot" may actually be columnaris, in fact that very likely is what is wrong with your fish. Columnaris is a very serious infection that needs treatment right away with broad spectrum antibiotics.
If you can post pics of the fish, as close up and clear as possible, it would be helpful.
Until you can, please describe exactly what the areas look like.
I don't remember if in your post you mention that you have quarantined the sick fish, but this would be very important to prevent the spread of illness to others in the tank.
Also, pristine water is very important to help them fight off whatever this is, so do big, frequent water changes to get those nitrates to safe levels.
We're pulling for you and wishing you success in helping your fish get well. Keep us posted.
Please, provide close up pictures and/or a very detailed desription of the lesions.
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