Are you sure its fin rot? It could be aggression.
Very hard to make any intelligent suggestion based on your post. Need to know much more about your system, paramiters, and other fish. Please read the sticky on how to post in this forum so we have some information to go on.
What's up with all the 'please read the sticky on how to post'.
Wasted time that could have been used more productively. That could have been said after you gave some humane advice.
I have read this SO many times today. Some one pass out crabby
That said...he wants you to post all your test results.....good to post water temp also and list your fish. If it's fin rot...your fish will show some signs of illness before long, if it's aggression from other fish...the fins will grow back. In some cases the 'biter' does such a good job he'll take so much of the tail off it will cause infection. I saw this in another aquarium at the lfs, a pj carndinal basically had a huge chunk not only out of the fin, but deep into his tail section. make sure it's not that deep. Hard to watch for aggression as sometimes it happens in the night while we sleep. Good luck. I know this doesn't help much....do look up fin rot and see if you can find some pics, there is a big difference between a 'bitten' fin and fin rot...and the biten fin will grow back quickly.