Simply put, over-stabilization occurs when too much cyanuric acid is added to pool water. Over-stabilization is a problem because cyanuric acid (CYA) can slow down chlorine’s ability to kill pathogens and algae. To explain this, you need to know that hypochlorous acid (HOCl) is the active form of chlorine in pools. The concentration of HOCl is affected by both the pH and the CYA concentration. Figure 1 shows the well-known relationship between pH and HOCl.