Fin rot and nipped fins can look identical. I recently had what I thought was a bacterial fin degeneration issue but it turned out to more likely be fin nipping at night by my nocturnal woodcats.
Any fish prone to nipping can often do so very sneakily. I was watching an Emperor Tetra one day, attempting to nip the Congo's fins. Emperors are not "nippers" like Black Skirts are, but given the temptation many tetra will try it, once anyway. It was quite an ambush tactic. I watched the tank for over an hour, and the Emperor gave up after maybe 40 or 45 minutes. I was tempted to net it out, but I didn't; I've never seen this again, and the five male Congo all have intact fins and caudal extensions, so I am assuming the Emperor has shown no further interest.
I would observe the tank very carefully; sit absolutely motionless in front of the tank for a good period, say an hour or more. This is not as silly as it may sound; fish do react differently when someone is in front of the tank, for one thing they associate it with food so their minds will be focused more on eating, plus with movement outside the tank there is a natural instinct to be cautious. I often see things I would never otherwise observe when sitting motionless [and I sometimes dose off
]. The video was quite short, but I did spot some of the Black Skirts "eyeing" the Bala and Congo, and that is the first step. You get to notice how they sort of sidle up near the target fish, waiting the chance.
If it is the Black Skirts, it will be during daylight as they are diurnal and thus inactive during total darkness. Perhaps the RTS, and I believe this species also is diurnal.