Stabiliser Levels in Aussie Pools: Get the Balance Right

Cyanuric acid (CYA) — commonly called pool stabiliser or conditioner — is one of the most misunderstood chemicals in pool care. Get the balance right and your chlorine works efficiently for weeks. Get it wrong and you'll either burn through sanitiser in days or create a pool that refuses to stay clean no matter how much chlorine you add.

What Stabiliser Actually Does

UV radiation from the sun destroys free chlorine rapidly. On a hot, sunny Australian day, an unstabilised outdoor pool can lose up to 90% of its chlorine within a few hours. Cyanuric acid forms a temporary bond with chlorine molecules, shielding them from UV degradation and dramatically extending their working life.

The catch: that same bond slows down how quickly chlorine can sanitise bacteria and algae. Too much CYA and your chlorine becomes effectively "locked up" — present on a test strip but unable to do its job. This is called chlorine lock.

The Right Target Range

Pool Type	Recommended CYA Level
Outdoor chlorine pool	30–50 ppm
Outdoor pool in high UV regions (QLD, NT, WA)	50–70 ppm
Indoor pool	0–20 ppm (minimal sun exposure)
Salt water chlorinator pool	50–70 ppm
Pool using stabilised chlorine tabs/granules	Test frequently — CYA climbs fast

Always aim for the lower end of the range if your pool gets partial shade. Higher is not better.

How to Test Your CYA Level

Basic 5-in-1 test strips give a rough CYA reading but are notoriously inaccurate at higher concentrations. For a reliable result:

• Turbidity test kit — the most common method; you dilute pool water and add a reagent, then check when a black dot disappears. Inexpensive and reasonably accurate.
• Drop-based reagent test kit — more precise than strips; available at most pool shops for around $30–$60 AUD.
• Professional water analysis — many pool retailers offer free water testing. Worth doing at the start of each season and mid-summer.

Test CYA at least once a month during summer and once at the start of the swimming season.

Raising Stabiliser Levels

If your CYA reads below 30 ppm in an outdoor pool, add a dedicated stabiliser product (pure cyanuric acid). Avoid confusing this with stabilised chlorine — products like trichlor tablets and dichlor granules already contain CYA and will raise your stabiliser level over time.

How to add stabiliser:

1. Calculate the volume of your pool in litres (length × width × average depth × 1,000).
2. Use the dosage guide on your product — typically around 1 kg of stabiliser raises CYA by roughly 10–12 ppm per 50,000 litres.
3. Pre-dissolve the granules in a bucket of warm water before adding to the pool.
4. Add directly to the skimmer box or broadcast around the pool while the pump runs.
5. Run the filter for at least 24 hours before retesting — CYA dissolves slowly.

Lowering Stabiliser Levels

This is where it gets frustrating. There is no chemical product that removes cyanuric acid. Your only practical options are:

• Partial drain and refill — the most effective method. Drain 20–30% of pool water and replace with fresh water. Each 25% water change roughly reduces CYA by 25%.
• Dilution over time — splash-out, backwashing and rainfall gradually dilute CYA. In Queensland's wet season this can happen naturally.
• Prevent it climbing in the first place — if you use stabilised chlorine products exclusively, CYA will build up season after season.

If your CYA is above 100 ppm, chlorine becomes so inefficient that you'll need massive doses to maintain sanitation. At that point, a significant drain and refill is the only sensible fix.

Common Mistakes to Avoid

• Using stabilised chlorine as your only sanitiser year-round — dichlor and trichlor add CYA every time you dose. Switch to unstabilised liquid chlorine (sodium hypochlorite) for routine top-ups once CYA is in range.
• Ignoring CYA when troubleshooting algae — if your chlorine level looks fine but algae keeps returning, test CYA before buying more shock. Chlorine lock is a common culprit.
• Adding stabiliser directly without dissolving — undissolved granules sitting on the pool floor can bleach or etch the surface.
• Assuming indoor pools need stabiliser — they don't. CYA in an indoor pool only hampers chlorine efficiency with no UV benefit.

Actionable Takeaway

At the start of this swimming season, do a proper CYA test with a turbidity or drop-based kit — not just a strip. If you've been using stabilised chlorine tablets or granules for several years without a partial drain, there's a good chance your CYA is well above the ideal range. A 25–30% water change costs relatively little and can restore chlorine efficiency overnight, saving you money on chemicals all summer.