you have all those fish in a 220 with what sounds like a tank that wasn't completed cycled.
that tank isn't going to house all those fish for life.
tank also probably under-filtered with two cans only and the bioload on the tank.
that's definitely a water quality issue, probably exacerbated by the sting, which maybe why she is the only one showing symptoms, if your nitrite is high then your tank isn't cycled and your other fish will decline eventually...