For those of you who occasionally have this problem, here are a few possible reasons. Please remember that this information is generic. Your pool may have different requirements. See your pool dealer if you have any questions.
The sun will cause Chlorine levels to quickly dissipate if you do not have the proper level of Stabilizer. The proper amount of Stabilizer in your pool water will extend the life of your Chlorine. Cyanuric Acid is the chemical used as a swimming pool stabilizer.
The proper range of Stabilizer using chlorine as the sanitizer, should be in a range of 30-60 PPM (parts per million). For the customers that use Granular Chlorine, that product has no stabilizer built into it. You need to use Cyanuric Acid on a regular basis to extend the life of your chlorine. If you add granular chlorine to your pool in the morning and get a chlorine reading, that reading could be zero in the afternoon due to low stabilizer level.
For customers that use Chlorine tablets, stabilizer is built into the tablet and Cyanuric Acid may not be needed as often. The problem is when a chlorine floater is used and 3” chlorine tablets are placed in the floater, the pool may not be getting enough chlorine or stabilizer because these tablets do not dissolve quickly enough. This will result in algae and almost a zero stabilizer reading. Using an Automatic Chlorinator with the chlorine tablets inside, will give you a better choice of your pool receiving the proper chlorine level and the proper stabilizer level. It is very important to get your pool water checked on a regular basis for the proper reading of stabilizer.