I started out using the API test kits which included tests for the most common parameters such as Alk, Ca, Ammonia, PH, Nitrite, Nitrate. After using it for awhile I discovered that it was very hard to get an accurate reading since it is all color based and a lot of the color shades looked the same to me. I have since invested in Red Sea test kits and are happy with them.
I am doing weekly tests for Ca, Alk and Mg and abut every 3rd week for PH, Ammonia, Nitrite, Nitrate etc. These values has been fairly stable for some time now and does now change much if anything at all.