What CYA (cyanuric acid) does — and why your pool needs it.

Cyanuric acid— CYA, sometimes called stabilizer or conditioner — binds to free chlorine in the water and slows the rate at which UV sunlight burns it off. Without CYA, an outdoor chlorine pool can lose most of its free chlorine to a single sunny afternoon; with CYA in range, the same chlorine holds through the day and still has enough left to do its sanitizing work overnight.

The binding is mild and useful inside the recommended band, and increasingly limiting as CYA climbs past it. At 30 ppmthe relationship is sun protection — the payoff the product exists for. At 80+ ppm the same chemistry starts to work against you: free chlorine shows up on the test strip but a growing share of it is bound up and unavailable for sanitizing, so algae can gain a foothold even when the readings look fine. The recommended band of 30 – 50 ppm sits where CYA is doing its job without trapping the chlorine it was meant to protect. Full definition lives in the CYA glossary entry.

CYAexists to solve an outdoor-pool problem: UV load. The moment a pool sits in direct sun, free chlorine is being broken down by the same light that's heating the water. The recommended range for outdoor chlorine pools and salt pools is 30 – 50 ppm— enough CYA to slow the burn-off without locking chlorine up. The same CYA glossary pill carries that band; matching it to the pool type matters because indoor pools and bromine pools have a different math.

An indoor chlorine pool, or any pool under a solid cover for most of the day, gets almost no UV load. Free chlorine doesn't burn off in the same way, so CYA is optional — many indoor operators run 0 ppm on purpose to avoid the lockup effect entirely. A bromine pool doesn't use CYA at all: bromine is intrinsically more UV-tolerant than chlorine and doesn't need a stabilizer to hold through a sunny day. If your pool is indoor or runs on bromine, the CYA reading is a parameter you don't really need to manage, and adding a stabilized product would be the wrong move.

Stabilized chlorine products add CYAevery time they dissolve. Trichlor tabs, dichlor sticks, and trichlor-puck feeders all carry CYA built into the puck or stick so the chlorine they deliver is already stabilized against UV. Stabilized shock (the “chlorine shock” bucket at the store) adds CYA for the same reason. Every dose adds a measurable amount to the reading.

Liquid chlorine (sodium hypochlorite), cal-hypo, and unstabilized shock do not add CYA. They sanitize the same water without leaving stabilizer behind, which is why a pool owner who has been feeding tabs for two seasons and switches to liquid chlorine usually sees the CYA reading stay flat for a while — it isn't coming down on its own, but at least it isn't going up. The pattern that pushes CYA past 100 ppmmost often is two or three seasons of trichlor tabs in a row, with no single large dose along the way — just the slow accumulation of stabilizer from every puck that ever dissolved in the water.

In a traditionally-chlorinated pool, raising the CYA reading high is annoying but recoverable. A big shock slug lifts the free chlorine concentration, and at the higher level of free chlorine enough remains usable to sanitize the water even at elevated CYA. The reading still says “high CYA” — but the dose pattern can sometimes power through it. A salt pool doesn't have that lever. The salt cell produces chlorine continuously at whatever percentage the controller is set to, and the controller has a hardware ceiling on what it can output. There is no shock dose-equivalent unless you add chlorine manually on purpose.

That makes CYA harder to manage in a salt pool, not easier. As CYA climbs past 80 – 100 ppm, more and more of the cell's chlorine output gets bound up and is no longer available for sanitizing — so algae can take hold in a pool whose test strip says the salt and the chlorine are both fine. The deeper explanation of why the salt cell can't just “turn up” to compensate for high CYA lives in the Salt Pool Guide; the short version is that this is the variable most worth staying on top of in a saltwater setup.

No residential chemical reliably lowers CYA. The stabilizer doesn't evaporate with the water, doesn't get burned off by UV the way chlorine does, and isn't consumed by the sanitation reactions it's part of. CYA only leaves the pool water when the water itself leaves the pool. The practical fix is a partial drain and refill: drain a fraction of the pool, refill with fresh water, retest, and repeat until the reading sits back in band. Where the budget tolerates it, a service-side reverse-osmosis treatment can pull CYA down without draining the whole pool, but it's a service call rather than a backyard chemical step.

The shape of the move matters. Total-fill refills are wasteful and usually unnecessary — a partial drain that targets a known drop in CYA reading is the common DIY path. There isn't a single correct percentage; pool size, current CYA reading, and target CYA reading all feed the math. The safer posture is to drain in increments and re-test between them rather than commit to a full refill on the first attempt. Check local disposal rules before draining — many municipalities restrict pool drain water from going into storm sewers or street-side drainage.