That assumes that the test kit is accurate and giving a true reading.
It's unlikely that the plants are taking up that much and that the fish are producing that much, small fish load with CO2 etc.
The CO2........tyhat can easily slow plant growth down and induce all sorts of algae issues, 15-30ppm is a wide range and hopely it's 30ppm most the day.
If you want to look and test a lot, focus on that, you'll get a lot more out that than testing for NO3.
The best NO3 levels for plant growth are 20-80 ppm for aquatic plants.
This goes back to Krombholz and Gerloff, 1966. I figure out this same range about 8 years ago and just recently ran across it. Paul was worried about the higher NO3 as folks have cows about adding NO3 and think it's some toxic death stew for fish(there's no evidence that it is, but NH4 is quite another matter, yet we still have fish........) and no one listened to him in hobby for some reason.
So 20-30ppm is ideal, water changes keep it from building up. From about 20ppm down you really drop the growth rate. 5 ppm for examples produces about 4.5 times less dry weight than 20ppm with non limiting conditions for the other parameters. Quite a bit.
If you change 1/2 the water, you should have 7.5 ppm according to that kit.
So do one and see.
I bet it's not much different.