One of the common causes of pool water turning green is due to the extreme high level of Cyanuric Acid (> 80 ppm) presents in the water.
Let me explain how the Cyanuric Acid is added to the pool water. Stabilized Disinfectants – Trichloroisocyanuric acid which is also known as TCCA, Trichlor or Stabilized Chlorine are used to sanitize the pool water.
Some of the manufacturers:
Shikoku Chemicals Corporation (Neo-Chlor)
Nissei (Nissan TCCA-90 / Hilite)
ICL Industrial Products (CDB)
Lonza / Arch Chemicals Inc (HTH)
TCCA contains Cyanuric acid prevents chlorine degradation due to the ultraviolet rays from the sunlight. Once the chlorine of the iso has been depleted, the Cyanuric Acid remain in the pool water. In topical region, sunlight can be so strong that the chlorine level drop by 90 percent within 2 hours without the use of Cyanuric Acid.
For convenient sake, some of the pool contractors use only TCCA instead of Calcium Hypochlorite. After 6 months to 12 months of continuous dosage of TCCA, Cyanuric Acid level will build up and exceed 80 to 100 ppm. Extreme level of Cyanuric Acid will locked up the chlorine completely and the chlorine will not sanitize the water. As a result, the pool water will turn green quickly as algae will grow rapidly under the hot sunlight.
For optimum chlorine efficiency, the Cyanuric Acid should be maintained between 30 and 50 ppm. some of the common test kits you can purchased on the market are:
Pentair Cyanuric Acid Test Kit R151226
Hanna HI 3851 Cyanuric Acid Test Kit
LaMotte Cyanuric Acid Test Kit 6838
Hach Cyanuric Acid Test Kit CY-3
Lovibond Cyanuric Acid Minikit AF422
Taylor Cyanuric Acid Test Kit
AquaChek Cyanuric Acid Test Kit
The only solution is to dilute the pool water with fresh water to lower the Cyanuric Acid level.