Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If you suspect the NO3 is in the 'unreadable red' zone, then dilute the tank water with as much RO as you suspect will give you a readable result.
For example, if you think the NO3 might be between 20-40 ppm, then see what 50% tank water + 50% RO will give you. Double the test reading to assign a value to the tank water.
You could try as little as 1ml tank water + 4 ml RO, then multiply the result by 5.
Alternate idea: If the test is anywhere in the red zone, do a 50% water change and retest. Maybe gotta do another 50% if it is still unreadable. Then let it stabilize over night, and test the next day.
2 x 50% water changes back to back is 75% water change. If the NO3 is still over 20 ppm after this, it means that it had been over 80 ppm which is WAY too high.
Does not really matter what the actual value is. If the test is red, do a big water change.
Now, back to dosing.
Fish food can contribute quite a lot of N, P and most traces.
Water changes can supply the Ca and Mg needed by fish and plants, as long as the GH is at least 3 degrees.
Fish food is low in carbon, potassium and iron.
Skip dosing anything and do 50% water changes, still run the lights and CO2 for a few days.
Then test the NO3.
Use this as a guide to how much fertilizer the fish food is giving to the plants.
If it is still readable, the NO3 can be a stand-in for P and the traces (but not iron).
Next, dose K and Fe.
After a few days test the NO3. The additional elements that fish food is low in might be just enough to kick the plants into using up more NO3.
If it is low, then start dosing KNO3, KH2PO4, and traces. You could use the EI method, or perhaps cut the recipe in half if you think these are being supplied pretty well by fish food. Whatever level you decide to dose the KNO3, also adjust the KH2PO4 and traces the same way.