Do Indoor Pools Need Cyanuric Acid? Pool Stabilizer Guide

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Quick Answer

No, indoor pools typically don't need cyanuric acid (stabilizer) because there's no UV light breaking down your chlorine. However, if you're experiencing unusually fast chlorine loss even indoors, maintaining a low level of 20-30 ppm CYA can provide some benefit without the drawbacks of higher levels.

Why Indoor Pools Are Different

Cyanuric acid's primary job is protecting chlorine from UV degradation. In outdoor pools, sunlight breaks down free chlorine rapidly - sometimes within hours. This is why outdoor pools need 30-50 ppm of CYA (or 70-80 ppm for salt water generators) to maintain proper sanitization.

Indoor pools don't face this UV challenge. Your chlorine naturally lasts much longer indoors, meaning you'll use less chemical and save money. This is one of the major advantages of indoor pool ownership from a maintenance perspective.

When Indoor Pools Might Benefit from Low CYA

There are specific situations where adding a small amount of cyanuric acid to an indoor pool makes sense:

• High bather loads: Commercial indoor pools or residential pools with frequent heavy use may benefit from 20-30 ppm CYA
• Temperature fluctuations: If your indoor pool experiences significant temperature swings, stabilizer can help maintain more consistent chlorine levels
• Ventilation issues: Poor ventilation that allows some UV exposure through skylights or large windows
• Chlorine demand problems: If you're constantly fighting chlorine loss despite proper FC levels and no algae

The Problems with Too Much CYA Indoors

Many pool owners make the mistake of treating their indoor pool like an outdoor one, leading to CYA levels of 50+ ppm. This creates several problems:

Chlorine Lock Issues

High cyanuric acid levels bind up your free chlorine, making it less effective. According to the FC/CYA relationship chart, you need higher chlorine levels as CYA increases. At 50 ppm CYA, you need 4 ppm FC minimum, but at 20 ppm CYA, you only need 1.5 ppm FC.

Algae Growth Risk

Overstabilized pools are prone to algae blooms because the chlorine becomes sluggish. You might test 3 ppm free chlorine but have poor sanitization due to high CYA binding the chlorine.

Wasted Chemicals

You'll spend significantly more on chlorine trying to maintain higher FC levels that high CYA demands, even though you don't need the UV protection indoors.

Testing and Maintaining Proper Levels

Testing CYA Levels

Use a quality test kit like the Taylor K-2006C to accurately measure cyanuric acid. The turbidity test requires proper mixing and timing - add the reagent, cap and invert 20 times, then read when the black dot disappears.

Test CYA monthly for indoor pools, or immediately if you're experiencing chlorine loss issues. Many pool stores use test strips that are notoriously inaccurate for CYA readings.

Target Levels for Indoor Pools

• No CYA: 0 ppm is perfectly fine for most indoor pools
• Low CYA: 20-30 ppm maximum if you determine you need some stabilizer
• Free Chlorine: 1-3 ppm depending on your CYA level and bather load

Adding Stabilizer to Indoor Pools

If you determine your indoor pool needs cyanuric acid, add it carefully:

1. Calculate dosage: 1 pound of stabilizer per 3,000 gallons raises CYA by approximately 30 ppm
2. For a target of 20 ppm in a 20,000-gallon pool: (20,000 ÷ 3,000) × (20 ÷ 30) = 4.4 pounds
3. Pre-dissolve granular stabilizer in a bucket of warm water - it dissolves slowly
4. Add the solution near return jets with circulation running
5. Wait 24-48 hours before testing, as CYA takes time to fully dissolve and register
6. Retest and adjust if needed

Safety note: Always add chemicals to water, never water to chemicals, and use proper ventilation even indoors.

Maintaining Indoor Pool Chemistry

With little or no CYA, your indoor pool chemistry becomes much simpler:

• pH: Maintain 7.4-7.6 for comfort and chlorine effectiveness
• Total Alkalinity: 80-120 ppm (60-80 for salt water systems)
• Free Chlorine: 1-3 ppm (lower end possible with minimal CYA)
• Calcium Hardness: 250-350 ppm for plaster pools, 150-250 for vinyl/fiberglass

Removing Excess CYA

If your indoor pool has high cyanuric acid from previous outdoor use or over-treatment, you'll need to dilute it. CYA doesn't break down naturally and can only be removed by:

• Partial drain and refill (most effective method)
• Reverse osmosis treatment (expensive but preserves water)
• Bio-Active or CYA reducer products (limited effectiveness)

Calculate the dilution needed: if current CYA is 60 ppm and you want 20 ppm, you need to replace about 65% of your pool water.