Nitrates of 10ppm would not have killed the fish. My tap water has 10ppm right from the tap. In fact, a normal fish tank should have some level of nitrates... Unless it is densely planted and the plants are bottoming the nitrates out.
I was more concerned with the nitrItes. What is the actual reading for them? I use polyfiber in my filters (cut to fit) so what I do is cut it in half when I want to replace it (which isn't often). I replace half, wait a few weeks then replace the other half. Never had any problems doing it this way.
I had a DG when I first started out, it was my favorite fish for a long time. I added some red-eye tetras to the tank (w/o QT'ing them) and they wiped out half my tank, DG included.

I learned my lesson about QT'ing the hard way. I never got a new one because it seems the stock has gotten weaker and weaker as of late.