2 black skirt tetra back fin is chipped off rest of the fish like neon tetra elephantfish and pleco is fine ?

Obviously there was a rumble. Black skirts can get moody, so maybe it was picking on someone and that someone in turn nipped the fin. It will probably grow back. However, just be sure to keep an eye out as sometimes (not always), the fish with the torn fin can develop a bacterial infection where the fin was torn (you didn't say exactly where). Make sure you do proper maintenance and water changes on your tank, and you shouldn't have any problems. Good luck!! ;o)