If your new to reefing or your tank is rather young you should test for PH,Alk.(dkh),Calcium,and Ammonia about ounce a week.After the tank matures,like when it gets to be about a year old, you get to know what your tanks needs are and the tank becomes more stable so then you might test ounce a month or so.Mostly now that my tanks are 2 and 3 years old I only test for DKH,and Calcium maybe ounce a month.If I see something that doesn't look right than I test for Nitrates, Ammonia,Nitrites,PH,DKH.,and Cal.