If this is HLLE (not an expert), you may want to remove the carbon, and add to the fishes diet. Put some Nori in the tank for the tang, and you can start soaking the food in vitamins to help.
This is just some information I found online from a previous expert here.
HLLE (Head & Lateral Line Erosion) - Updated 6-9-2023 What You Need To Know: * This is more of a condition than a disease, and is usually reversible if the erosion is not too severe. * Possible causes include: Stray voltage, Lignite Carbon, nutritional deficiency and copper exposure. Severe...
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