9. When Is The Best Time To Add Chlorine? The best time to add chlorine varies with the type of chemicals that you are using. It is best to follow the manufacturer's recommendation in this regard. If you are using our recommended product, the best time to chlorinate is whenever it is convenient for you because the sun has very little effect on this type of chlorine in your swimming pool. Definitely chlorinate any time your test shows a chlorine residual below 0.6 ppm.