At the minimum, you need a test kit that can accurately check pH and “free chlorine” in the pool water. To accurately check “free chlorine”, the test kit should use DPD tablets or liquid. “OTO” (yellow liquid) type chlorine test kits are difficult to use to accurately determine free chlorine. The test kit must be able to test for free chlorine from 0.5 to 3 ppm (parts per million). However, it is suggested that test kits with a higher free chlorine range (to 5 or 10 ppm) be used. If pool stabilizer is used, cyanuric acid must also be checked, at least once per month. Cyanuric acid test kits are available but you can also take a water sample into some pool supply companies once per month. Most test kits will also check for combined chlorine, total alkalinity, and calcium hardness, which are useful for properly maintaining a pool.