the issue with your fish could have simply adding too many too fast. i dont know how fast you added them, so thats just a guess. it could also be that the onset of brook in the other persons tank was coincidence. brook is one of those that you would generally recognize if you saw it, so if you didnt see brook signs on your dead fish, perhaps that was just coincidence.
if you have nitrate levels that dont match between multiple tests, i would take the average of what the results were. chances are, you are significantly lower than the 80 rreading you got initially. i wouldnt freak out too much at the moment. if you get a different test kit, and the result comes back insanely high, then i would worry. on a side note, i will say that the red sea color chart for nitrate is good for the low end, but anything over 20, youre going to have a hard time determing the color to match. i find that anywhere between 0 and 10 is easy enough, but if it gets toward 20, it becomes hard to tell what color of pink it is. i dont know if salifert color charts are any better