In order of importance, they are pH, nitrate, phosphate, alkalinity, nitrite, ammonia and Calcium (for reef tanks, the calcium test kit is more important than nitrite and ammonia). A good pH test kit is critical, and an electronic pH monitor is even better. Ammonia and nitrite tests are only needed occasionally after cycling. A nitrate test kit is a good overall test for water quality after the tank becomes established. You should perform a pH test once a week and a nitrate test every two weeks. The other kits are not necessary, but may be needed to solve particular problems or after you advance to more delicate creatures.
