My first question would be, how old was she? I just went through this. I check my levels every day. Added some fish 2 weeks ago on a Friday. All went well, or so I thought. Was monitoring the levels with no spikes. Got Up Sunday and went out for brunch and when I cam back I had lost a Black Skirt Tetra. Immediately did a level check and all was well. All the rest of the fish looked fine. Of course for the next several days I was a little on edge but I did not lose any more. So I lost 1 new fish out of 6. Just one of those things so it does sometimes happen. If all tests well and the rest of your fish appear to be ok, then everything is probably fine.