CYA, TDS, and Why They Only Go One Direction
Some things in pool water evaporate. Others don’t. Understanding the difference explains why many Arizona pools eventually struggle—even with consistent service and “balanced” chemistry.
One of the most common questions we hear is:
“Why does this keep happening if the pool is serviced every week?”
The short answer is that some things in pool water accumulate over time, and once they’re there, they don’t leave on their own.
Two of the most important are CYA and TDS.
What is CYA (stabilizer)?
CYA, or cyanuric acid, protects chlorine from being destroyed by the sun. In Arizona, it’s essential—without it, chlorine burns off quickly.
The issue isn’t having CYA.
The issue is that CYA does not evaporate or break down.
Once it’s added to a pool:
It stays
It accumulates
It only leaves when water leaves
Over time, regular chlorination combined with evaporation causes CYA levels to slowly climb.
What is TDS?
TDS stands for Total Dissolved Solids.
This is a catch-all measurement that includes:
minerals
salts
treatment byproducts
dissolved compounds introduced over time
Every chemical you add contributes a small amount.
Every time water evaporates, those dissolved solids stay behind.
Eventually, the water becomes “crowded.”
Why this matters to how your pool behaves
As CYA and TDS rise, several things happen:
Chlorine becomes less effective
The pool becomes harder to keep clear
Small problems show up faster
Chemical adjustments feel inconsistent
You end up using more product for less result
This is why a pool can:
test “in range”
still look dull
still struggle to hold chlorine
still need constant tweaking
The chemistry hasn’t failed — the water environment has changed.
Why this is more common in Arizona
Arizona pools experience:
extreme evaporation
long swim seasons
heavy sun exposure
frequent top-offs instead of full water replacement
This accelerates buildup compared to cooler or wetter climates.
It’s not poor maintenance.
It’s physics.
Why adjusting chemicals stops working at some point
Chemical adjustments are meant to fine-tune water.
They are not meant to:
remove accumulated stabilizer
remove dissolved solids
reset long-term buildup
When CYA and TDS get too high, adding more chemicals is like adjusting the thermostat in a house with the windows open — you’re fighting the system instead of fixing it.
How buildup is actually corrected
There are only two ways to reduce CYA and TDS:
Remove water and replace it
Filter dissolved solids out (via RO)
That’s it.
No product can selectively remove them while leaving the rest of the water intact.
This is why, at certain points, we recommend:
partial drains
water resets
mobile RO services
Not because weekly service failed — but because the water has reached the end of its usable lifecycle.
How we use this information during service
When we evaluate a pool, we’re not just looking at today’s numbers.
We’re asking:
Is the pool responding normally?
Is chlorine behaving predictably?
Is filtration doing its job?
Is buildup starting to limit effectiveness?
When the answer becomes “no,” we stop chasing numbers and start talking about restoring the water itself.
Coming next
If you’re wondering what a water reset actually involves—and how we decide when it’s the right move—we break that down in the next article.