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.
GH test's do not tell you whether the reading rendered is comprised of mostly calcium,or magnesium without specific testing for both.
Why wait until deficiencies appear, when you can add a little of everything that plant's need? Nutrient's in the way of mineral salt's are not the boogey man.
Might read up on how important magnesium is to plant growth.
Also would not rely on fish food ,fish waste,for KNO3 and phosphates for everyones stocking,feeding's, types of food and their content,are often different.You would be just guessing in my view.
Also as plant mass increases,the plant's may very well use more of everything so EI dosing ,or scaled back version for low tech, is much more productive than trying to limit this or that nutrient, at least until such time as you are able to readily recognize what plant's are trying to tell you.
If you are dosing KNO3,and KH2PO4,then as mentioned,,you do not need the K2SO4 but it does no harm to add it either.
Water changes are entirely up to whoever is performing them but for me,,If I've already got the hose out ,python,bucket's, then 50 % water change is just as easy as smaller water changes and plant's and fishes will thank you.
