Compare the pricing and active ingredients to what you could buy here from nilocg or from Planted Aquarium Fertilizer - Home
or other supplier.
You want separate packages so you can customize the dosing.
1) KNO3 is the most common supply of nitrogen and potassium. If your tank already has high nitrates from fish food then you probably won't use much of this.
2) KH2PO4 supplies phosphorus. It is dosed in too low an amount to count for potassium, though there is some in here. This was skipped in older fertilizer schemes like PPDD because they thought phosphates caused algae.
3) K2SO4 supplies potassium and sulfur. See 1). If you do not want to dose much KNO3, then you will use this to supply potassium.
4) Trace minerals: The most common product used is CSM+B. (Note- the Plantex name is old. The product is now called Nutritrace)
Plantex CSM + B is composed of 1.50% magnesium, 0.10% chelated Copper, 7.0% chelated iron, 2.0% manganese, 0.06% molybdenum, 0.40%
Zinc, 0.04% boron.
5) Some people add additional chelated iron.
6) GH booster. Sigh!
GH is both calcium and magnesium.
Plants use Ca and Mg in a ratio of about 4 parts Ca: 1 part Mg.
There is no sense in just adding magnesium unless you already know your tap water is low in magnesium and has enough calcium.
If the GH of your tap water is at least 3 German degrees of hardness, then there is probably enough. If you want to boost the amounts, then you could add a product like Seachem Equilibrium or Barr's GH booster to raise the GH by 1-2 degrees. Check with your fish. Soft water fish prefer low GH.
You certainly do not need to pay inflated prices for Epsom salt (the most common source of magnesium sulfate in such packages). Get it at the local dollar tree or 98 cent store if you really need it. Yes, there are some tap waters that are low in magnesium, but high in calcium. Not very many, though!