I honestly do not trust any NO3 test kit, especially either of those brands without calibration for NO3.

I just ran a set of calibration solutions against the API for a Marine tank client, I threw it out. They now use a Lamotte and have a set of known standards.

I've also been showing others for many years the problems with NO3 test kits.
I see no mention of NO3 calibration so I can only assume it was not done, 99.9% of all cases with hobbyists it's not and even then, it's not done correctly.

The API read 20ppm, without any NO3 and with 40ppm.

Try adding 10ppm of NO3 via KNO3 and wait and see.
Obviously there is some NO3 in there, and perhaps a fair amount in soil you added.

As soil ages, it gets mineralized, namely from an organic form of N to NH4 to NO3.
You generally will have very little in the water column after awhile unless you add it.

So you very well might not have any in the water column but why would that help?
If you we stopped adding any ferts, is that good?
No.

Having NH4, not NO3 in the water column is the problem.
Again, a very low level is fine. Just not a lot like in the start up phases.

But why is low NO3 all that great?
If you assume so, you need to add it to show this and make sure that other factors do not influence your results.


Adding Excel will take care of the Carbon.
Adding flourish will adress the traces.
So GH and PO4, K are what's left.

Adding some GH booster/PO4 will take care of that and then you can isolate and add KNO3.

You do not need that much, try dose 2x week, 5ppm of NO3 via KNO3.
This will help the growth and health of the plants.

Plants will switch to water column and back to root and back to water column uptake given time. The transition periods can cause some die back and change.
Using the excel kills the algae so all you see is namely poor plant health for a little while until, they adapt.

Plants cannot run away, they have to make do and adapt with what they have.

Regards,
Tom Barr