I now have two black-skirt tetras who have torn fins, one with the tail completely nipped off. Water parameters in their 30 gallon tank (with 6 black-skirt tetras and 6 cories, all about an inch) are 0 ppm ammonia, 0 ppm nitrites, 0 ppm nitrates. The light was off for about a week while I ordered a new bulb online, and that's when this damage happened, although it may be so extensive just because I didn't notice. Now the two damaged fish are in a 10 gallon that formerly housed my betta (who has been relocated for the purpose) which has parameters of 0 ppm ammonia, 0 ppm nitrites, 10 ppm nitrates. I added aquarium salt (1 tablespoon) to the 10 gallon. Both tanks are at about 78 degrees.
One of the black-skirt tetras (the one with no tail) is not eating, but they're both swimming fine.
So: Are the white spots related to the injury? Is this definitely a fin nipping problem, or is it possible it's crazy aggressive fin rot? And what can I do to heal the fish/ stop them from doing this to each other?