Can you post pictures of all the fish in the tank, as well as the tank and the rock/ ornaments in it?
Fungal infections normally cause problems to damaged tissue. If the fish is developing fungal infections regularly, it could be fighting and being injured by another fish, or it could be scratching itself on something in the tank.
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Doing big regular water changes and gravel cleaning the substrate can help reduce the number of disease organisms in the water, and this can reduce the chance of secondary infection getting into wounds.
I recommend doing a 75% water change and gravel cleaning the substrate every week.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Clean the filter at least once a month and wipe the inside of the glass down each week when you do the water change.