Finrot or what?

Yesterday I noticed my male kribs tail was tattered, as though the membrane between the bones had been lost, as with finrot. But there was no blood on his fin, none of the other fish show any symptoms, and I did a 30% water change the day before so water conditions are good. Could it be that the female krib attacked him? they seem to have a sort of love/hate relationship. Also in the tank are 5 black phantoms and 2 upside down cats, neither of which would attack a krib. i added so tetra finrot treatment yesterday evening, and this morning I found that the tattered area of the kribs tail had gone completely, as though it had ben bitten. Most of his tail is still there though and he doesn'yt seem to be having much trouble swimming.

Any ideas?

thanks,
momar