You probably don't have 80 ppm of nitrates. Unless you calibrate your nitrate test kit you can't be sure what the readings mean. It could be a technique error, a bad batch of test kits, an aging issue with the test kit, etc. Calibrating will show you what the results really mean.
You didn't mention how much light you have. That is the most significant parameter, because it is light that determines how fast the plants can grow, therefore how fast they use up nitrates, CO2, and other nutrients.