Stabilised chlorine adds CYA every time. In small hot tubs that climbs fast. Here is how to spot, manage, and prevent CYA creep.
Cyanuric acid (CYA) protects free chlorine from UV breakdown but binds it loosely so it acts more slowly. In a 30,000-litre swimming 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, at which point your sanitiser becomes sluggish.
Trichlor tablets contain about 57% CYA by mass. Every time one dissolves into your tub, it adds roughly that much CYA in proportion.
Dichlor granules are about 56% CYA. The same calculation applies, just delivered faster.
Calcium hypochlorite and sodium hypochlorite contain no CYA. Lithium hypochlorite contains no CYA either. These are the unstabilised options.
Test CYA monthly with a turbidity tube (filling the tube with reagent until you cannot see the black dot at the bottom).
If CYA has climbed by 10 ppm or more in a month, you are over-using stabilised products and need to switch the routine.
The most common indirect sign is slow chlorine response. You add the right dose, the reading climbs, but combined chlorine builds within hours. That is CYA at work, binding most of your free chlorine.
Fix: partial drain (one third) and refill. There is no consumer chemical that removes CYA economically.
Prevent: switch to unstabilised chlorine for routine maintenance dosing. Keep a small pot of dichlor for the rare moment you need a fast top-up but reach for unstabilised by default.
For outdoor tubs that benefit from some UV protection, deliberately dose 20 to 30 ppm CYA at refill (with a one-off measured amount of stabiliser) and then switch to unstabilised top-ups for the rest of the cycle. You get the protection without the creep.
Yes, after about a year from opening. Old reagent gives optimistic (too-low) readings. Buy fresh annually.
Yes. The chemistry and the test are identical. The pool kit reagent often comes in larger quantities at lower per-test cost.