Fin rot is generally caused by stress, no? How big is the fish/your aquarium? pH level? Anything else that could have stressed it out?
If it's really bad, consider treating it with an antibiotic.
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~Drew
10G Nano * 10G Sump * Deltec MCE 600 Skimmer * JBL Viper 150w MH * Zeovit * Vortech MP40W
Millions long for immortality who do not know what to do with themselves on a rainy Sunday afternoon. ~S. Ertz

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