Using my 10g as an example...
1/8 tsp KNO3 is recommended minus half = 1/16 tsp.
1/64 tsp of KH2PO4 is recommended.
If you wanted to get the other 1/16 tsp of KNO3 that you lost you likely would
need to up the KH2PO4 at least to 1/16 tsp.
This would be way over the recommended amount of it.
Since it is where the Phosphates come from, you don't want that.
The K2SO4 adds something that is needed and not in the KNO3 anyway.
It would be the normal substitute for KNO3.
In fact, look on here...
and see that it is added as an "optional" on the list of nutrients.
In my tank since I don't have much plants, if I do use the 1/8 tsp
that is recommended it gives me a red test result on Nitrates
when tested just before water change. So the plants haven't use
enough of it. So if I dose 1/32 tsp of KNO3 and 1/32+1/16 tsp of K2SO4
then the test just before the water change in light Orange or about 10-15 ppm
on the nitrates. That way I'm getting my full 1/8 tsp for the total.