If you cant test for it,dont add it.
I dont use any kind of supplements except for buffer and calcium.The ONLY reason I even dose those is because my SPS corals are eatting it up pretty quick.
great rule of thumb there Yote. I agree. If you can't measure it or test it, don't add it to your tank. If you use high quality a salt mixture, that will take care of a lot of problems - Oceanic or Kent Marine.