You only need to test GH, KH and PH. Aquarium Pharmaceuticals has good kits at a reasonable price for those. The first two determine the value by a sudden change from blue to yellow, and the PH kit uses a little chart of colors you try to match your test tube sample to. They aren't precise measurements, but they don't have to be. As long as you verify that the KH is above 3, and that you have GH a bit higher or more, then all that's left is to find out how much CO2 you are putting into the water. For that you need PH. Just take a sample of the tank water, let it sit in a shallow glass for a few hours, then measure the PH of that water and the water in the tank. There should be a .8 to 1.1 difference in the two measurements if you have 30 - 45 PPM of CO2 in the water. If you use the EI fertilizing method, which is simply adding enough of all of the fertilizers to be sure you never run too low, then changing 50% of the water once a week or so to get rid of any excess, you don't need to measure any other parameters. Doing so is interesting, and if done carefully can help with troubleshooting, but isn't really essential.