I believe the problem is solved. I got paranoid and checked the ammonia level in my tank - it was at 1 ppm... it should be at 0. New tanks do blossom up to 6 ppm, but, I think mine should be beyond that point.
I just did a quick 50% water change.
I think Ms. Swordtail got gill damage from ammonia poisoning. It looks as if she's gasping for air.
The other fish seem healthy... so, that is good. But I think Ms. Swordtail may have been my first lesson learned.