Keep the NO3 level as low as possible. Certainly under 10ppm, lower if possible.
Monitor the Loach with the sore mouth. Is he eating? Is the swelling going down? Is the white getting larger, or growing fluffy? Is there other signs of infection, such as red streaks radiating away from the mouth?
Look up pictures of fish diseases and see if you can match what is going on with your Loach.
Happy fish keeping!
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