Honestly, I don't test for anything but nitrates. In an established tank, you'll never see ammonia or nitrites again and pH and kH you only need to test for if you start to inject co2. When the tank is first set up, it is probably a good idea to test pH, kH, gH and nitrates, but ammonia and nitrite tests are a waste of money. Once your tank cycles, you'll never use those two again unless you set up another tank completely. Plus, if your tank is heavily planted, the plants should suck up any nitrogen before it becomes a problem.