Your test said you had from 20 to 110 ppm of nitrate in the water. Only the very upper end of that is getting near where it could be a problem for the fish. The bigger problem is that without calibrating the test kit first, you can't tell if the readings mean anything at all. It is 50-50 that the reading from the test kit is way off.
Planted tanks need nitrates, which is why we dose them. If you just stop dosing for a couple of days the plants will consume what is in the water and bring the concentration way down. I'm assuming you have growing plants in the tank, with enough phosphates and other nutrients, plus carbon, for the plants to actually grow well.
Finding a dead tetra has nothing to do with nitrates. If you had no plants in the tank, and it was a fish only tank, it would be a different thing, but planted tanks must have nitrates or the plants just don't grow. (Blue green algae will certainly grow well though.)