Pool stabilizer (CYA): safe ranges, warning signs & fixes

Costa Rica’s strong UV radiation can burn off chlorine quickly, especially in outdoor pools with full sun exposure. That’s why “stabilizer” is often recommended. When used correctly, cyanuric acid (CYA) can improve chlorine stability.

The issue is that CYA doesn’t get consumed like chlorine—it builds up over time. When it gets too high, chlorine can become less effective, water becomes harder to keep stable after rain or heavy use, and you may experience recurring algae and repeated “corrections” that don’t last.

This guide explains what cyanuric acid does, when it makes sense in Costa Rica, and how to avoid the most common (and expensive) scenario: chlorine reading present, but sanitation performance weak.

What cyanuric acid is—and why it’s used

CYA reduces the rate at which UV light breaks down free chlorine. Think of it as “sunscreen” for chlorine.

This matters in Costa Rica because:

• UV exposure is strong most of the year.
• Pools are used year-round (not seasonal).
• Organic load is frequent: rain, leaves, dust, sunscreen, and heavy use.

A properly managed stabilizer level can reduce dramatic daily swings and improve consistency.

What stabilizer does (and what it doesn’t)

It does:

• Help chlorine last longer in sunny outdoor pools.
• Reduce daytime chlorine loss.
• Improve consistency when everything else is correctly designed.

It does not:

• Fix algae on its own.
• Replace filtration, circulation, or routine cleaning.
• Work better “the more you add.”

When it makes sense in Costa Rica

• Outdoor pools with strong direct sun
• Vacation rentals where consistency matters
• Pools using trichlor/dichlor tablets (key point: tablets add CYA over time)

When it becomes a problem

CYA becomes a problem when it accumulates to a point where chlorine acts slower and sanitation performance drops. In Costa Rica this is common because tablet use is often continuous and CYA is rarely tested.

Common signs

• Algae appears even when chlorine “tests okay”
• Water collapses quickly after rain or heavy use
• You need higher and higher doses to maintain clarity
• Chlorine feels “locked” or “not doing much”

Why CYA builds up (the common causes)

• Continuous tablet use (trichlor is the #1 source of unnoticed buildup)
• Adding stabilizer without testing first
• Minimal water replacement over long periods

Common mistakes to avoid

• Using tablets year-round as the only sanitation strategy
• Treating algae by only adding more chlorine without checking CYA
• Ignoring filtration and circulation (especially in rainy, high-organic environments)
• Mixing multiple chemicals without a clear plan

Practical approach (keep it simple)

• Make CYA part of your periodic testing routine.
• If you rely on tablets, do it intentionally and monitor buildup.
• Treat water issues as a system problem: chemistry + hydraulics + environment.
• When instability repeats weekly, stop guessing—check CYA and overall system sizing.

Final recommendation

In Costa Rica, cyanuric acid can be very helpful because it protects chlorine from intense sun. But once it accumulates, it becomes one of the most common drivers of unstable water and recurring algae.

The answer isn’t “never use it” or “add more.” The answer is straightforward: test it, control it, and align your chlorine strategy with tropical conditions.