Chlorine is broken down by the sun's UV rays, which means that you may need to add an unnecessary amount of chlorine. To protect the chlorine, you can add cyanuric acid, some chlorine tablets contain cyanuric acid. Then there is usually no need to add more cyanuric acid. You should be careful not to add too much, the only way to reduce the level is to replace parts of the water.
If you have any type of automatic disinfection, check the manual for the recommended level of cyanuric acid.