Actually that is not quite correct. TPN does contain K and Mg (potassium and magnesium). TPN+ adds in the phosphorus and nitrogen. I have been using TPN and it predecessor Tropica MasterGrow for almost a decade now. However, I also dose individually phosphorus, nitrogen, potassium and occasionally iron all from SeaChem as needed.
I tried making a homeade version of the Tropica juice for a few months and went back to using the Tropica. I started with it because they are the worlds pre-eminent aquatic plant folks. If the stuff is good enough for them, who I am I to argue. The only knock anybody can have regarding the TPN and TPN+ might be cost.