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If you want to do less water changes (smaller volume, for example) then yes, use less KNO3.
Also, use less KH2PO4. Fish food supplies reasonable amounts of N and P.
But I would not reduce the K2SO4. When you reduce the KNO3 that will reduce the potassium. Fish food is not a good source of potassium.

What is the GH of the tap water?
GH measures Calcium and Magnesium.
Plants use Ca and Mg in a ratio of about 4 parts Ca to 1 part Mg.
It is good to have water with more or less about that ratio.

If the tap water has a GH of at least 3 German degrees of hardness, then you can assume there is enough Ca and Mg for the plants. If you see some problems, then you can do separate tests and figure out what you need (Ca by itself, Mg by itself or a combination).
I would not dose Mg by itself without making sure that is what the tank needs. Too many fertilizer companies sell people magnesium when they do not need it. Epsom salt is cheap. But it is not a complete GH booster. It has not calcium.
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