Both the above seem right but perhaps some added info might be worthwhile. This is a time when I like to go to dry ferts as a cheap way to add just one item rather than a pre-set with several items I may not want. Sometimes we get into situations where it seems dry is difficult to figure but I fell that is because we get way too hung up on being "exact" when what we are doing is always going to be an educated guess we sometimes call an ESTIMATE. Since what I add is always going to be an estimate, I don't sweat too hard over getting it exact as estimate and exact don't come in the same thought.
Perhaps it is time to order up a bit of dry potassium nitrate and add a pinch of so until you see something show on the testing. This is my spot for cheap ferts:
Dry Fertilizer | Aquarium Fertilizer
A one pound bag of nitrate is $3 but when we add shipping it may seem like a bite but compared to using pre-mixed where we pay a bunch for the water, I like the dry and a pound will last a long, long time.
An older post that gets into the exact question that you have with lots more detail about what I use: