Nitrates from 5ppm to 100+ppm in a day?

[Kungpaoshizi]

Good to see it was the test kit, my first thought was your bacterial masses have crashed or food got dumped in.. :/

 

[Randy Holmes-Farley]

In many kits, nitrate is reduced to nitrite (with cadmium metal or other reducing agent) for the actual detection detection. Oxygen may also be reduced and so may deplete the reducing agent if it enters the kit in sufficient quantity.

The chemistry is described here:

http://www.open.edu/openlearn/scien.../test-kits-water-analysis/content-section-3.2