Many aquarists encounter it at some point: fin rot. An annoying fish disease, but luckily definitely not the worst thing that can happen to your fish. In most cases, it should clear right up if you take the right measures!
Find out how to treat (and prevent) fin rot on Aquariadise here.
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PS.The photo was provided by the fish blogger who went by theblondeaquarist – anyone know if they left entirely or just changed their URL? I would like to credit properly.