Cyanuric acid is the stabiliser in most chlorine tablets. It protects chlorine from the sun, which is why stabilised chlorine lasts so much longer in an outdoor pool. But it builds up over time, and too much of it makes chlorine work more slowly.
Quick answer: if you use Chloro Maxx 90 TCCA tablets or granules, you are already adding cyanuric acid with every dose, so you rarely need to add more. If chlorine stops working well even at normal doses, the stabiliser may have built up.
What is cyanuric acid?
Cyanuric acid, formula C3H3N3O3, is a white powder that bonds loosely with chlorine in water. That bond shields the chlorine from the sun's UV light, so less of it burns off on a hot day. Pool professionals call it stabiliser or conditioner.
Stabilised and unstabilised chlorine
| Type | Contains cyanuric acid? | Zyax product |
|---|---|---|
| TCCA (trichloroisocyanuric acid) tablets and granules | Yes | Chloro Maxx 90 tablets, granules |
| Calcium hypochlorite shock | No | Hypo Pool Shock |
| Chlorine-free sanitiser | No (contains no chlorine) | Pool Maxx 35 |
Why too much is a problem
Cyanuric acid is not used up like chlorine. Each dose of stabilised chlorine adds a little more, and it only leaves the pool when water is drained or splashed out. As the level climbs, chlorine becomes slower to kill algae and bacteria, so the pool turns cloudy or green even though the chlorine reading looks fine.
Signs to watch for: algae that keeps coming back, cloudy water despite normal chlorine readings, and needing more and more chlorine to get the same result.
How to keep it in balance
- Test it. Use a test kit or strips that read cyanuric acid. Standard 5-in-1 strips read chlorine, alkalinity, hardness and pH, but not cyanuric acid.
- Dilute it. If it is too high, drain part of the pool and refill with fresh water.
- Alternate your shock. Use unstabilised calcium hypochlorite for shocking, so you are not adding more stabiliser every week. See calcium hypochlorite for pools.
- Indoor pools get little sun, so they need little or no stabiliser.
For daily dosing, see how much chlorine your pool needs.
More pool care guides
Frequently asked questions
What does cyanuric acid do in a pool?
It shields chlorine from sunlight, so the chlorine lasts longer in outdoor pools.
Do I need to add cyanuric acid separately?
Usually not if you use stabilised chlorine. TCCA tablets and granules such as Chloro Maxx 90 already add cyanuric acid with every dose.
Can there be too much cyanuric acid?
Yes. It builds up over time because it does not get used up. Too much makes chlorine work more slowly, so algae and cloudiness come back more easily.
How do I lower cyanuric acid?
The usual way is to drain part of the pool and refill with fresh water. Then switch to an unstabilised shock such as calcium hypochlorite for a while.
