A chlorine demand problem occurs when the free active chlorine (FAC)---the chlorine available as a sanitizer---in a swimming pool can't be sustained at the required level despite regular maintenance and shocking. The problem occurs when most of the chlorine added to the water reacts with organic matter or other chemicals to form compounds called chloramines. Even though the total chlorine content in the pool is high, the FAC is consistently too low because the chlorine is bound to the chloramines. The cure usually involves adding very large amounts of shock chemicals to the water to remove the chloramines, and then following up with routine shocking.
Use chlorine test strips to determine the FAC and total chlorine levels. Dip the strip in the pool water, and compare its color to the test kit chart.
Subtract the total chlorine value from the FAC value to determine the chloramine level. Shock the pool with chlorine shock to reduce the chloramine level and test the water again. If the FAC value remains low and the chloramines high, suspect a chlorine demand problem.
Take a sealed sample of your water to a professional pool chemicals dealer or water testing service. They have the lab equipment needed to differentiate between water that has a chlorine demand problem and water that needs a different adjustment.
Follow the lab's recommendations on treating the water. Chlorine demand problems usually require the addition of 10 to 20 times the normal dosage of chlorine or mono-pursulfate shock chemicals.
Shock the pool weekly with chlorine after the FAC level has stabilized.