Maracyn is a gram positive antibiotic while the most fish infections are gram negative. I would opt for maracyn2, triple sulfa or furan2 (depending on what you have available to you). Just keep an eye on your parameters if you treat with antibiotic because it may affect your good bacteria. Another option you can try (only if your comfortable handling your fish without undue stress) is topically applying a product such as BioBandage or methylene blue once a day.