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I'd try a better test kit than a cheapy.
Also, use known standard reference solutions to check if the test kit is accurate, this is what we and every researcher does at the lab..........we calibrate the system just like you do a pH probes etc...every time we do a large run or even a single test.
If you wanna a long time series, you can freeze 20-50ml water samples(easier than calibrating every time) and thaw and test many all at once. Goes much faster and you see longer trends and patterns, often seeing things most folks would miss if they test just once a week without calibrating things.
I'd try reducing the NO3 as best you can, eg lots of water changes.
The rest of the tank will love that, cost $ though.......
Tom
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