When pool water changes from a clear blue to a murky green, the cause is typically one of two distinct problems: a rapid biological infestation or an invisible chemical reaction. This discoloration signals that the water’s delicate balance has been lost, allowing unwanted elements to take over. Understanding the difference between these two mechanisms—living organisms versus dissolved minerals—is the first step toward restoring a safe and clear swimming environment.
Algae Bloom: The Most Common Reason
The most frequent culprit for green pool water is the uncontrolled proliferation of algae, which are microscopic, plant-like organisms. Algae spores are constantly introduced by wind, rain, and swimmers, but they only bloom when the environment supports their rapid growth. Warm water temperatures, especially above 80 degrees Fahrenheit, combined with ample sunlight, create the perfect breeding ground.
The actual bloom is enabled by a lack of effective sanitation, particularly a low level of free chlorine, the active form of disinfectant. Chlorine’s job is to destroy these spores before they multiply, but when its concentration drops below the minimum threshold, algae can quickly turn a pool green in a matter of hours. This type of green water is usually cloudy or opaque because of the sheer volume of suspended organisms.
While green algae is the most common and fastest-growing type, other varieties signal a more persistent problem. Yellow, or mustard, algae appear as a dusty, yellowish-green film, often clinging to shaded walls and surfaces. Black algae, a type of cyanobacteria, manifests as dark, stubborn spots that have developed a thick, protective layer, making them highly resistant to standard chlorine treatment. All types feed on nutrients like nitrates and phosphates, which enter the pool from sources such as lawn fertilizer and organic debris.
The presence of algae indicates a failure to maintain a consistent residual of active sanitizer in the water. The growth is fueled by warm conditions and a nutrient supply, but the absence of sufficient chlorine allows the spores to colonize and overwhelm the water’s clarity. A pool that is green and cloudy almost always points to this biological invasion.
The Role of Metal Oxidation
A second, less common cause of green water is purely chemical: the oxidation of dissolved heavy metals. These metals, most frequently copper and iron, are invisible when dissolved in the water, but they precipitate out of solution and become visible when they react with an oxidizing agent. The green color results from the suspended metal compounds, not a living organism.
Copper is the metal most likely to create a green tint, often resulting in a clear, translucent blue-green hue. Copper typically enters the water through the source water, from copper-based algaecides, or from the corrosion of pool equipment, such as a copper heat exchanger. Iron, another common metal, will oxidize into a rust-colored or brown tint, though it can sometimes contribute to a greenish appearance.
The critical trigger for this chemical discoloration is often the addition of a high dose of oxidizer, such as a chlorine shock treatment. The shock reacts with the dissolved copper ions, causing them to become oxidized. This reaction creates insoluble, colored metal compounds that cloud the water and can eventually stain pool surfaces. If a pool instantly turns green or blue-green immediately after shocking, and the water remains clear rather than cloudy, metals are the likely cause.
Underlying Water Chemistry and Filtration Issues
Both algae blooms and metal oxidation are often symptoms of systemic failures in the pool’s underlying chemistry and mechanical systems. The most influential chemical parameter is the water’s pH level, which dictates the effectiveness of the sanitizer. Chlorine is most potent when the pH is maintained within the ideal range of 7.2 to 7.6.
When the pH rises above this range, the active sanitizer, hypochlorous acid (HOCl), converts into the much less effective hypochlorite ion (OCl-). For instance, at a pH of 7.5, chlorine is approximately 50% effective, but at a pH of 8.0, its effectiveness drops to about 20%. This chemical inefficiency leaves the water vulnerable to algae growth.
Another factor is the concentration of cyanuric acid (CYA), which acts as a stabilizer, protecting chlorine from being degraded by the sun’s ultraviolet light. Maintaining the correct balance is delicate, as too much CYA will also reduce the chlorine’s sanitizing power. Levels above 70 parts per million can significantly slow down chlorine’s ability to kill contaminants, allowing algae to thrive.
Beyond chemical balance, mechanical issues like poor water circulation and filtration also contribute to green water problems. If the pool’s pump is not running long enough or the filter media is dirty, the water will not be adequately exposed to the sanitizer or filtered of microscopic debris. This allows organic matter, which serves as a food source for algae, to accumulate and prevents oxidized metals from being physically removed.