Frontosa fry has black on fins?

[Manafel]

Hello, I woke up this morning to see one of my frontosa fry had been picked on, but instead of the fin being the normal color, the surrounding area is black. Is this possibly just dead tissue or some kind of disease? My water params are ammo:0 nitrites:0 nitrates:30 and ph of 8. Here's a picture.. Thanks

 

[Carnal]

Since the poor little thing has been being picked on, those are just stress marks and should be gone once the constant aggression stops.