Swimming Pool Shock: The Ultimate Pool Owner’s Guide

When to Use Pool Shock: The A-B-Cs of Shocking Pool Water

Why do you need to shock a pool? Pools are shocked for a few different reasons. One is to remove combined chlorine molecules, aka chloramines, from the water. Another common reason to shock a pool is to remove excess bather waste and bacteria after heavy pool use, contamination events, or to treat visible algae in the water. Bromine-treated pools and spas also use pool shock to reactivate bromide ions in the water.

There are really 3 main reasons to shock a pool or spa, raising the Free Available Chlorine level high enough (10–30 ppm) to oxidize or destroy the offending contaminants. We like to call these the A-B-Cs of Pool Shock.

Algae: Green, yellow, pink, or black, the best algaecide is chlorine, and lots of it. Pool algae growth can be controlled with algaecide. But to kill algae and clear the pool, you’ll need to use pool shock. A pH on the slightly low side of the ideal range, or around 7.2–7.4, will allow the chlorine shock to be most potent. Some types of pool shock, including cal-hypo and liquid chlorine, will raise the pH slightly. Stabilized dichlor shock has a near-neutral pH. Depending on the severity of the algae, 10–30 ppm chlorination is needed to kill active algae blooms. Chlorine accelerators, such as Yellow Out work to boost your chlorine level to fight severe algae blooms of all colors.

Bacteria & Bather Waste: Bacteria can enter the pool from many sources, and most of it is harmless. However, pathogenic bacteria may also exist. Use chlorine shock to remove bacteria after extensive pool use, severe storms, heavy rainfall, long winters, or to address swimmer “accidents.” Bather waste includes any contaminants brought into the pool by swimmers, including dead skin cells, hair, lotions, cosmetics, and soaps, as well as more potent contaminants like sweat, fungus, and yes, even urine and feces.

Chloramines, Contaminants, Cloudy Water: Combined chlorine molecules are responsible for “red eye” swimmers and a strong chlorine odor. When chloramine levels exceed 0.3 ppm (Total Chlorine – Free Chlorine = Combined Chlorine), add enough chlorine or non-chlorine shock to break apart the combined chlorine molecules. This is usually around 10–20x the tested CC level. As mentioned in the previous paragraph, contaminants in the water are a reason to shock the water so the pool stays sanitized. And last, but not least, cloudy pool water is often a symptom of poor water balance and low sanitizer levels. Balance the water and shock the pool to get the water crystal clear again.