Swollen gill

One of the rainbow females has had this for quite some time. I thought it was due to the little spike of nitrites I discovered weeks ago, and hoped it would heel after NO₂ would disappear, but I'm afraid it's actually worsening. Otherwise she's doing fine, and today even manage to jump first after the archers shot a fly, and got it!She kept it in her mouth for minutes, chewing it but being unable to swallow it (fortunately!), until one of the males took over when she spitted it for a second.