Cyanuric acid (CYA), often called ‘stabilizer’ or ‘conditioner,’ protects free chlorine from rapid breakdown under UV/sunlight exposure — without it, outdoor pools would lose chlorine within a few hours of sun exposure. This makes some CYA genuinely useful for any outdoor pool.
The problem is that CYA only accumulates over time — it isn’t consumed or broken down the way chlorine is, so every dose of stabilized chlorine product adds to a total that only rises unless the water is diluted. Above a certain concentration, CYA reduces how effectively the remaining free chlorine can actually kill bacteria and algae, even if the chlorine reading itself looks ‘normal’ — a condition sometimes called chlorine lock. This is a common, underlying cause behind pools that seem to have ‘normal’ chlorine but keep turning green.
Because safe/target CYA ranges vary by regulation, product, and pool type, they should always be confirmed against your test kit or chemical supplier’s guidance rather than assumed from a single generic figure.