As I keep a fish-only tank, I do not feel that >20 NO3 will cause massive die off. I once own a tankset with the NO3 is constantly at 50-80+. Fishes doing well.
A bacteria boom could cause a massive die off though. If i remember correctly, bacteria take in a large amount of oxygen thus there is a danger of de-oxygenation in the tank.
If anyone want to try, add 1 cup of sugar in to ya tank. :P
Joking.