The main point here was plant consumption!
The Results: Heavily planted tank
Average NO3 uptake per day 3-3.5ppm.
Average Fe uptake per day .015ppm.
API NO3 kit I've never had an issue.
I've reduced this to a 1ml water sample with 2 drops of each reagent.
If color is negligible I do a 2nd test cut with 50% RODI.