Okay, without seeing a good photograph, it's very difficult to say if your fish has got fin rot, if indeed it has. A fish's fin is extremely delicate, they can get damaged very easily and often do. People often think they're fish as that this condition when they see damaged fins. I've seen some of my fish with fins that are almost shredded. They have always healed very nicely.
Fin rot is caused by bacteria infection which can be brought on by the poor water conditions. Make sure that your water is in good condition and do some water tests, ammonia, nitrite, nitrates and pH. Please post the results for us.
Like I said before, please post some photographs. I have got some fish health books with some very good photos of fin rot.