No, KNO3 is not a filler, it's just a nutrient.
ETDA and other items allow the metal salts functionality.
Try dissolving pure Fe, a nice cast iron version.
Then try dissolving FeSO4.
You need that type of compound, it's not just "filler".
Pure solid elemental forms are dangerous, hard to use, dissolve etc.
Potassium metal is explosive/burns when added to water.