My honest guess was either a disease of some sort or the BN - the wounds are not deep. My question is the wound directly due to the BN (if it is a BN); or due to infection? I don't see any obvious signs of infection but I'm not sure what i would see if there is an infection. I have a particularly large plain male BN that never comes to the front during feeding time and that would be the one I suspect but is there any way i can 'prove' that is the cause of the damage ?Caveat....I have yet to own Angels......but....
BN's are known to attach to a slabsided fish on occasions where the chance presents itself, such as a resting Angel, to enjoy the slimecoat. They do it at night when the slabside fish is static/resting. The removal of slimecoat can trigger infection and will inevitably leave open sores which other fish can then nibble at. BN's are very similar to the Chinese Algae Eater in that if the opportunity arises they will latch onto a dozing fish after dark. The more times that the BN latches onto the fish, the weaker it will become and the BN learns that its available without much resistance.
Not every BN will do it but they are known to be opportunistic in that regard. I would not dismiss the BN as the culprit so easily.
The wounds on the Angel are fairly identical in size and shape/depth for a sniper BN slimecoat attack. When the fish loses slimecoat in that way it does leave angry sores which can encourage other fish to tuck into the now open raw flesh.
Definitely do not dismiss the BN. The L204 is purely veg orientated whereas the BN does have carnivorous traits. Dismiss my theory if wished but my money is on the BN doing sniper slimecoat removals at night.