After your tank is cycled, NO2 and ammonia isn't really needed so sits on the shelf. Planning corals, Ca and Alk plus Mg, fish only don't need. NO3 is good to have. Phosphate, from reading seems really hard to measure accurately with the usual aquariums kits. pH, used to watch when setting up a Ca reactor using a test kit but I had a problem reading so got a pH meter but really don't worry about system pH.
Also don't go blow the money on the big Hagen kit like I did when first started.
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