When you setup your tank new you will be testing almost daily watching for it to cycle, Ammonia to spike then drop, followed by Nitrites, and then Nitrates. After that you will only test for nitrates in a normal run of things until you get into corals and you'll start testing PH, Cal, Alk, MG and Phosphates for example. Right now I test Cal and Alk daily (just started 2 part dosing so trying to fine tune consumption), Nitrates about every 2 weeks, and Mg and Phos monthly. I'm thinking to drop my nitrate test to monthly provided I keep up my weekly water change and feed consistent volumes.