Chlorine, in the form of hypochlorous acid, is the most effective means of keeping swimming pool water safe. A chlorine residual of 0.5 parts per million will kill disease bacteria. A higher residual will help to keep algae in check.
Algae cause unsafe water turbidity and stain the pool shell.
Excessively turbid pool water can prevent your seeing someone unconscious on the bottom. For chlorine there are two popular methods, the OTO and DPD test kits. The kits look similar, but they rely on different chemicals. Your pool supplier will be able to provide you with either. You should also test for the pH of your pool water.