Maintaining a minimum level of chlorine in your pool or spa is important for keeping the water clear, discouraging algae growth and killing harmful bacteria that can cause disease. Ideally, the amount of "free chlorine" in the water should be no less than 1.0 parts per million (ppm). There are, however, a variety of chemical reactions taking place that tend to lower the amount of chlorine available to sanitize and protect your pool and the people who use it.
The first step to ensuring proper chlorine levels is having and using a test kit at least once a week. The most effective kits are those that can measure all the various chemical components of pool water, including chlorine, pH, alkalinity and stabilizer. Additionally, make sure the reagents in the kit are not expired as old reagents can provide inaccurate readings. If in doubt, replace your test kit every year. Once you're sure your test kit is accurate, there are a variety of other factors that can cause low chlorine levels.
The pH value is the single most important element in pool water chemistry because it affects the balance of every other chemical in the water. Chlorine doesn't work well in pool water with either low or high pH values. If the pH is less than 7.2, the chlorine dissipates quickly and you get a low chlorine reading. In water with pH higher than 7.8, the chlorine becomes inefficient and you again get a low chlorine reading.
Closely related to pH is alkalinity. If total alkalinity is too high the pH becomes difficult to adjust, which directly affects chlorine levels. Alkalinity should be kept between 80 and 150 ppm.
Stabilizer, also known as cyanuric acid or conditioner, protects chlorine from being degraded by sunlight. Just as with pH, both high and low levels of stabilizer can result in low chlorine levels. Stabilizer levels lower than 30 ppm don't provide enough protection from the effects of sunlight so the chlorine levels drop quickly. On the other hand, levels above 50 ppm cause the stabilizer to completely surround the chlorine atoms and make them inactive.
In addition to the chemical factors, chlorine levels tend to be lower in pools that receive lots of direct sunlight, have higher water temperatures and are heavily used.