I agree with all Romad has said, and it *would* be best if you had any way to get an image up for us to look at.
Fin rot is one of those funny things that can present in a variety of ways, darkened edges around the torn edges is one, but it can also look more like frayed edges - basically, the edges of the fins are disintegrating, so fin-rot won't generally look like a clean break - while a fresh nip or an injury will.
Even if the injury was caused by a nip or a tear, fin rot is something to be on the lookout for, as it is a bacterial infection that can easily affect a torn fin if it gets the chance. Fungal infections are also likely to take advantage of a situation like this, so even if you don't see it now, keep a close eye on things as they heal. Also be on the lookout for any redness in the fin.
The GOOD news is that, given a low-stress environment and clean water, most torn/ripped/nipped fins will heal up very quickly on their own. Keep the water extra clean, and keep a close watch on the injury, and if all goes well it will heal up in no time and medication shouldn't be necessary.
I would highly recommend that you park yourself across the room from the tank with the overhead people lights out for a while and watch your tank. Keep very still, and the fish will forget about you - if there's a fin nipper in the bunch, you'll find him when they forget that you're there and resume normal behavior - this could take an hour or so, so be sure to have a comfy seat! Another option is to set up a video camera and leave the room, then scan for anything odd. If not a nipper, look to the ornaments in the tank, is there anything that the fish could have snagged or torn his fin on?
If you do see signs of fraying at the edges of the fins, or darkening, redness - or anything other than a clean break in the fin, it may be worth medicating. Finrot isn't something that is contagious, necessarily. Rather, it's a bacteria that is always present in our tanks, and doesn't generally cause a problem until poor conditions, stress, or a torn fin, allow it to get a foothold and cause trouble. . .
I hope this helps, and that you figure out what caused the injury and that it heals up very quickly for your poor little friend!
Last edited by Chesh; 02-25-2013 at 01:43 AM..