Things You'll Need
PH test kit
Pool chlorine test kit
Phosphate test kit
It's important to maintain the right level of chlorine to prevent the growth of bacteria and algae in a pool. Unfortunately, chlorine in pool water dissipates over time. The rate at which it does so depends on the pool's chemistry, exposure to sunlight and frequency of use. If you find the chlorine levels in your pool are falling faster than you'd expect -- so fast that it's difficult to maintain a desirable level of chlorine -- follow a few steps to pinpoint the problem.
Test your pool's pH. At high (alkaline) pH, chlorine is far less effective as a disinfectant; at low (acidic) pH, chlorine may dissipate more rapidly, so an over-acidic pH might contribute to your problem. An acidic pH will usually cause other problems as well, such as corrosion of metal pool pumps, rapid loss of alkalinity and eye irritation.
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Test for total chlorine and free chlorine. Free chlorine is the amount of chlorine present in the water as hypochlorous acid, the compound that kills bacteria. Total chlorine, on the other hand, measures both the free chlorine and the combined chlorine, the chlorine that has already reacted with other compounds and is no longer available to kill bacteria. If your pool has a high concentration of combined chlorines, the water will probably be cloudy and make your eyes sting but will do a poor job killing bacteria. Try superchlorinating the pool to rid it of the combined chlorine if this is your problem.
Look for algae. Algae growth in your pool will rapidly consume free chlorine. High temperatures, high levels of phosphates, dirty or clogged filters and low chlorine levels can promote algae growth. Scrubbing with a brush can remove algae from surfaces if they have accumulated in one place. If algae has become established in the pool you may need to kill it with an algaecide; these chemicals are available from many pool and home supply stores.
Consider using a stabilizer like cyanuric acid. Ultraviolet light breaks down free chlorine, so outdoor pools will lose chlorine more rapidly than indoor pools. Cyanuric acid will form a complex with free chlorine to prevent it from breaking down. It reduces the effectiveness of the chlorine but increases its lifespan. Once it has been added, cyanuric acid does not dissipate, so never add too much and carefully follow the manufacturer's instructions. Please note, however, that you should never use cyanuric acid on an indoor pool.
Swimmers should never use the pool during superchlorination.