Pool stabilizer is a chemical additive for pools that extends the effectiveness of chlorine. It works by slowing the breakdown of chlorine molecules by the sun's ultraviolet radiation.
Pool stabilizer includes cyanuric acid. The chlorine molecules form a weak bond with the cyanuric acid that makes the chlorine more resistant to UV radiation, but it also slows the rate at which chlorine kills bacteria and algae. Cyanuric acid does not decay in water and is not affected by UV radiation. This can cause cyanuric acid levels to accumulate and negatively impact the sanitation of the pool. Over-stabilized pools need to be drained partially to return cyanuric acid levels to optimal levels.