Tear In My Guppy's Fin

[michtank83]

I have a tux guppy who a few weeks ago when I was having nitrite trouble, was just floating at the top of the tank. Now my nitrite is down to 0 and the guppy has been more active swimming around the tank and etc. But today I notice a tear in the tail fin, and it looks like its been chewed on, I dont think its from the other fish cuz they want nothing to do with him. Is it fin rot, or something else that needs to be medicated.

Thank You

 

[africantigerfish7]

What other fish are in the tank?