Shocking a pool uses a high concentration of chlorine to oxidize organic contaminants and kill active algae blooms. Algaecide is generally a preventative agent designed to inhibit new growth over time. Performing these two maintenance steps in the correct sequence ensures that both chemicals can perform their intended functions effectively. The answer to whether you can add algaecide after shocking your pool is yes, but the timing is important.
The Conflict: Why Shock Neutralizes Algaecide
Never introduce algaecide immediately following a shock treatment. Pool shock is a powerful, fast-acting oxidizer, typically concentrated chlorine, designed to destroy organic matter. This high oxidizing power breaks down the active ingredients in the algaecide.
When algaecide (often polyquats or copper) is added to highly chlorinated water, the shock actively breaks down these chemical structures. It essentially sacrifices the algaecide molecules to the oxidation process, rendering the product useless. This means the product is wasted, and the pool owner must wait for the chlorine level to drop before adding a new dose.
Copper-based algaecides pose an additional risk of staining in a high-chlorine environment. High levels of chlorine can oxidize the copper, causing it to precipitate out of the solution. This can potentially leave unsightly black or blue-green stains on the pool’s plaster or vinyl surfaces.
Determining the Safe Waiting Period
The safe waiting period between shocking and adding algaecide is determined by the level of Free Available Chlorine (FAC) in the water, not by a clock. Algaecide can only be safely introduced once the FAC level has dropped to the normal operating range, typically between 1 and 5 parts per million (ppm). A concentration higher than this will continue to neutralize the algaecide.
Testing the water is a non-negotiable step before adding the second chemical. A reliable test kit or strip will provide the current FAC reading, indicating if the water is ready. This dissipation process often takes between 8 and 24 hours, but this time frame is highly variable.
The rate at which the chlorine level drops is heavily influenced by environmental factors. Direct sunlight rapidly breaks down unstabilized chlorine, causing it to dissipate faster. High water temperatures and organic contaminants will also consume the chlorine more quickly, potentially shortening the waiting period. Conversely, shocking the pool at night or having a high concentration of cyanuric acid (stabilizer) can prolong the time it takes for the FAC to return to a safe threshold.
Completing the Treatment Cycle
Once the FAC level is confirmed to be low enough, the algaecide should be added according to the product’s label directions, usually by pouring it slowly around the perimeter of the pool. Run the pool’s filtration system for at least 8 to 12 hours following application. This circulation ensures the algaecide is thoroughly distributed.
If the initial shock treatment was used to clear a visible algae bloom, the dead algae may be suspended in the water, causing a cloudy appearance. Brushing the pool walls and floor after the algaecide application helps push remaining particles toward the filter system for removal. This mechanical action is important for achieving a clear pool.
The final step is to retest the overall water chemistry, specifically the pH and alkalinity levels. The introduction of large quantities of chemicals can cause these parameters to drift out of their optimal ranges. Bringing the pH back to the ideal 7.4 to 7.6 range ensures maximum comfort for swimmers and maintains the effectiveness of all sanitizers.