By The Zagosa Team. Published .
Reviewed or updated .
Cyanuric acid, or CYA, shields free chlorine from UV breakdown but binds it loosely, so it works more slowly. In a 30,000-litre pool, CYA from stabilised tablets builds up over years. In a 1,200-litre hot tub, the same tablets can push CYA past 50 ppm in a single month, and at that point your sanitiser turns sluggish.
Where CYA comes from
Trichlor tablets are around 57% CYA by mass. Every tablet that dissolves adds roughly that proportion of CYA to your water.
Dichlor granules are about 56% CYA. The same maths applies, just delivered faster.
Calcium hypochlorite and sodium hypochlorite carry no CYA, and neither does lithium hypochlorite. These are the unstabilised options.
Spotting the creep
Test CYA monthly with a turbidity tube, filling it with reagent until the black dot at the bottom disappears.
If CYA has jumped 10 ppm or more in a month, you are leaning too hard on stabilised products and the routine needs changing.
The most common indirect clue is slow chlorine response. You dose correctly, the reading rises, but combined chlorine builds within hours. That is CYA at work, tying up most of your free chlorine.
Fixing and preventing it
To fix: a one-third partial drain and refill. No consumer chemical removes CYA at a sensible cost.
To prevent: switch to unstabilised chlorine for routine dosing. Keep a small pot of dichlor for the odd fast top-up, but reach for unstabilised by default.
For outdoor tubs that benefit from a little UV protection, deliberately dose 20 to 30 ppm CYA at refill as a one-off measured amount, then use unstabilised top-ups for the rest of the cycle. You get the protection without the creep.
FAQ
Does CYA test reagent go off?
Yes, after about a year once opened. Old reagent reads optimistically low. Buy fresh every year.
Can I use a pool CYA test on a tub?
Yes. The chemistry and the test are identical. Pool kit reagent often comes in larger quantities at a lower cost per test.