Swordtail with puffed gills and clamped fins

[Laser]

I had to euthanize the female swordtail that I got about a month ago. Last night she was struggling and kept getting stuck on the filter intake. Her gills were puffed out and her fins were clamped close to her body. The rest of her body seemed somewhat sunken inwards. I have no idea what I'm up against and none of the other fish show any signs of illness.

 

[Fishanatic]

I had to euthanize the female swordtail that I got about a month ago. Last night she was struggling and kept getting stuck on the filter intake. Her gills were puffed out and her fins were clamped close to her body. The rest of her body seemed somewhat sunken inwards. I have no idea what I'm up against and none of the other fish show any signs of illness.

Did you test you're water levels? Temperature? Oxygen?
Did she show any other signs of illness?

 

[Laser]

Ammonia and Nitrite - 0
Nitrate - around 20
pH - 7.6

It's a moderately planted tank in which I also run an airstone so I don't think the oxygen levels are of any concern. I keep the temperature at 80 degrees. The other fish seem fine including a male swordtail I bought at the same time.