Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
The general guide that I have seen suggests the NO3:PO4 ratio of about 10:1 is pretty good. If you do not have a PO4 test, then just dose per EI ratios.
If you are having too-high NO3 (heavily stocked tank, lots of fish food) and need to reduce the KNO3 dosing, then reduce the KH2PO4, also. Fish food has both N and P.
If you are dosing the full level of EI, there is supposed to be enough K from KNO3. (The tiny amount from KH2PO4 is so small it almost does not count). But if you have seen pin holes and are dosing a bit of K2SO4 or other potassium source that is just fine.
Next... If you need to reduce the KNO3 and KH2PO4, then I use K2SO4 to replace the KNO3, at the same rate. I think the dose is not exactly the same, to make the potassium come out the same, but is close enough to target, then monitor the plants to fine tune it.
The current recipe I am using is about 50% of the EI rate for KNO3, and the same amount of K2SO4.