learner's question

Basic chemistry: N is the symbol for nitrogen, O is the symbol for oxygen, P is the symbol for phosphorus, K is the symbol for potassium, H is the symbol for hydrogen. KNO3 is a chemical compound made of one atom of potassium, one atom of nitrogen and 3 atoms of oxygen. KH2PO4 is a chemical compound made of one atom of potassium, 2 atoms of hydrogen, one atom of phosphorous, and 4 atoms of oxygen.

We dose KNO3 for potassium and nitrogen, KH2PO4 for phosphorus. There is potassium in the latter compound, but we dose so little of it, compared to KNO3, that it doesn't provide much potassium.

Plants require nitrogen and phosphorus, as mentioned. They get it in the form of nitrates and phosphates, respectively.

You would not have much luck injecting gaseous nitrogen into your aquarium, nor would you have much luck placing phosphorus (red allotrope only please!) into your aquarium.