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Can’t Keep a Chlorine Reading on Your Pool or Spa?

Chlorine demand reflects an inability to maintain an adequate free chlorine residual in pool or spa water, even though the water is balanced and properly maintained. It can take the form of low total and free chlorine residuals or high combined chlorine (chloramines).

Chlorine demand is caused by many different contaminants entering the water that increase oxidation levels (the demand for shock) or tie up free available chlorine. Symptoms can include slimy or slick walls and cloudy water, off odors, or constantly testing very low chlorine even though you recently added. Water can also be clear and still have a high chlorine demand.

A sudden drop in cyanuric acid (and no leak or water loss) can be an indicator that there is a chlorine demand problem, as the CYA reading is often masked by the demand. If this occurs, do not add stabilizer.

Causes of Chlorine Demand

Specific causes of chlorine demand can include heavy bather loads, inconsistent maintenance, or external sources such as fertilizers, pesticides, pollen, or nearby construction kicking up a lot of airborne particulates. The following are some common causes and their explanations:

  • Resistant algae, fungus, or bacteria: An infestation can exhaust normal chlorine residuals and require a specially formulated algicide to solve the problem. Often, the growth is not visible on the pool surface itself. Pink slime and water mold are notorious for growing first inside lines, skimmers, and behind light niches before becoming visible. Growth in these areas can deplete chlorine steadily.
  • Nitrogen contaminationLawn fertilizers and other nitrogen products in pool water produce a high level of chloramines, which require larger amounts of chlorine for oxidation.
  • Source water: Lake water, which is usually contaminated with algae, metals, and other debris, should not be used to fill pools or spas. Well water may have some of the same problems. Even a local municipal water supply can create high chlorine demand, especially if it contains chloramines. In these cases, every time fill water is added to the pool, chloramine and nitrogen levels rise, requiring more chlorine. Shocking after fill water addition helps prevent excessive chloramine levels in this situation, as does avoiding the addition of large amounts of fill water at a time.
  • Rain and Pollution: Contaminants from factories, highways, airports, and other sources may be deposited in pool water, especially during rainfalls. Clouds sometimes transport pollution over long distances. Rain and wind also carry algae spores, leaves and other debris which raise chlorine demand. During the winter, stagnant water in uncovered pools exposed to air, rain and snow often develop chlorine demand problems.
  • High bather loads: A large crowd using the pool or spa over a few days can boost bacteria and oxidizable compounds in the water to unusually high levels. Requiring all swimmers to shower before bathing can help prevent this. This is especially true if bathers enter the water with a lot of “product” on their bodies or in their hair, i.e. moisturizers, lotions, sunscreen, or conditioner.  Since this is not going to be likely most of the time, shock immediately following parties or other occasions where many people have enjoyed the pool or spa.

NOTE: A zero chlorine reading does not necessarily indicate that there is no chlorine in the pool. The chlorine level may be so high that it is bleaching the color out of the reagent. If you suspect that the chlorine residual is high, dilute the sample with half distilled water and retest.

To Eliminate Chlorine Demand

For pools:  Shock the pool with 750gm of HtH Extra per 10,000 L of pool water. Circulate the water continuously after application. Three hours after adding HtH Extra, test the water for chlorine. If it is not greater than 3 ppm, repeat this step.

For hot tubs:  Shock with 30gm of Chlor Aid per 1,000 L of water. Circulate the water continuously after application. Three hours after adding, test the water for chlorine. If it is not greater than 3 ppm, repeat this step.

Continue shocking and retesting every three to four hours until you can maintain a 3 ppm free chlorine reading for 24 hours.


Drain a foot of water (spa) or 2.5 feet of water (pool) and dilute with fresh water to eliminate some of the demand. Always check environmental factors such as water tables etc. before proceeding with a drain. Consult the pool manufacturer or builder before draining significant amounts of water from any pool, and make sure the power is off before lowering the water level.

The best way to prevent chlorine demand is to follow a three-step program. Consistent sanitation, regular shocking, and the application of an algae preventative (pools) are the best weapons against any kind of problem, chlorine demand included.

To minimize the risk of chlorine demand from outside contaminants, shock after periods of heavy use and avoid using chemicals such as lawn sprays, fertilizers, pesticides and other pollutants if possible.NOTE: Always follow label directions and manufacturer’s instructions for each product used. Conditions may vary from pool to pool.

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