Slow spine arching in neon swordtail
I do some work for a LFS that has a single neon swordtail female that is slowly (over a month or more now) developing an arch to its spine. It was QT'ed as soon as I noticed it, but I also had a neon swordtail that developed this and eventually recovered with no real treatment. I'm not thinking it's TB as there seem to be no other symptoms, and it is progressing really slowly. Also no other fish in the tank it was in are showing any signs of Ill health. Since we all know a lot of fish experience high stress and ill health in many LFS I would expect if it were TB it would have spread to at least 1 other fish. Also I really really hope it isn't TB, TB in a LFS could be quite disastrous I imagine.
The water in both tanks at last test using an API liquid kit was 0 ammo 0 trites. Fluctuated between 4 tests of trates between what looked to be 0 and 20, eventually got two tests of approximately 12 if I remember correctly. I suspect the tests at 0 and 20 may have been contaminated somehow, or I dropped the reagent unevenly, I was feeling a bit sick that day. These are typical test results for his tanks, and anything else means we're doing immediate WC's even if it's off schedule.
So, I'm asking for any ideas what might cause this spine arching, there are no other signs of anything, no sores, lesions, no redness to gills, no rapid breathing or clamped fins, and its swimming is not noticeably affected even. Although its posture does look odd when it swims, well, yeah, its spine is curving... I don't know much about swordtails, only kept a pair before really and only for a few months. Is it possible this is something that can happen with them, similar to how balloon type fish are selectively bred to achieve their deformities? Any ideas would be appreciated, as I'd rather not cull the fish for no reason, and I can't leave it quarantined forever.
No matter how much I learn or know I'm still gonna be a NOOB, but that doesn't mean I'm an idiot!