CYA Too High? Your Chlorine Tabs Are the Problem

You've been putting chlorine tabs in the floater or the skimmer for three years because that's what the pool store told you to do. The pool looks fine — most of the time. But lately the chlorine doesn't seem to hold, algae keeps coming back even though you're adding the same amount of tabs, and the pool store tested your water and said your cyanuric acid is "a little high." They sold you a bottle of something that didn't help.

Here's what's actually happening: every chlorine tablet you've added to your Frisco pool for the past three years has been pumping cyanuric acid into the water. Your CYA is probably above 100 ppm — maybe above 150 ppm. And at those levels, your chlorine is essentially disabled. You're adding sanitizer that can't sanitize.

What Cyanuric Acid Is and Why Tabs Are the Problem

Cyanuric acid (CYA) is chlorine's sunscreen. It protects free chlorine from being destroyed by ultraviolet radiation. Without CYA, the Frisco sun would burn through your chlorine in 2-3 hours. With CYA at the proper level (30-50 ppm), chlorine lasts all day.

The problem is how CYA gets into your pool. There are two ways:

Way 1: You add it directly. Granular stabilizer (pure cyanuric acid) added to the pool at the start of the season. This is the correct way — you add a measured amount to reach 30-50 ppm and stop.

Way 2: Your chlorine tabs add it for you. Trichlor tablets (the standard 3-inch chlorine pucks used in floaters and chlorinators) are stabilized chlorine — they contain both chlorine AND cyanuric acid. Every tab that dissolves in your pool adds chlorine that gets used up, and CYA that stays behind permanently.

The math that nobody tells you:

One 8-ounce trichlor tablet adds approximately 2.5 ppm of CYA to a 15,000-gallon pool. If you use 2 tabs per week, that's 5 ppm of CYA added per week. Over one summer (20 weeks), that's 100 ppm of CYA added — on top of whatever you started with.

CYA doesn't evaporate. CYA doesn't get filtered out. CYA doesn't break down from sunlight or chemicals. The only way CYA leaves your pool is through splash-out, backwash, and drain water. For most Frisco pools, the natural CYA loss is about 5-10 ppm per year — a fraction of what tabs add.

The result: After 2-3 years of tab use, CYA climbs to 80, 100, 150, even 200+ ppm. And at those levels, your chlorine stops working.

Why High CYA Kills Your Chlorine

CYA protects chlorine from UV — that's good. But CYA also binds to chlorine molecules, reducing the amount of free chlorine available to kill bacteria and algae. The higher the CYA, the more chlorine is bound up and unavailable.

The relationship is not linear — it's exponential:

CYA Level	% of Chlorine Actually Available	Effective FC at 3 ppm
30 ppm (ideal)	~3%	Adequate — kills algae
50 ppm (acceptable)	~2%	Still works, slightly reduced
80 ppm (high)	~1.2%	Marginal — algae can establish
100 ppm (too high)	~0.9%	Chlorine barely works
150 ppm (way too high)	~0.6%	Chlorine is essentially disabled

At CYA of 150 ppm, your test kit says "free chlorine 3 ppm" — but the effective killing power of that chlorine is roughly equivalent to 0.5 ppm in a pool with proper CYA. Your pool shows chlorine on the test. It just can't do its job.

This is why:

• Your algae keeps coming back even though you're adding chlorine
• The pool turns green faster after rain or heavy use
• You're using more and more tabs but the results are getting worse
• The pool store keeps selling you algaecide and phosphate remover instead of addressing the real problem

How to Know If Your CYA Is Too High

Test it. Bring a water sample to a pool store and ask specifically for the CYA reading. Or use a CYA test kit at home (Taylor K-1000 or equivalent). The reagent test (where you look down a tube until a dot disappears) is more accurate than test strips for CYA.

Ideal range: 30-50 ppm Acceptable: Up to 60 ppm Too high: Above 70 ppm — chlorine effectiveness is compromised Way too high: Above 100 ppm — you have a problem that chemicals won't fix

If your pool store tests your water and says CYA is "a little high" but doesn't tell you the number, ask. "A little high" from a pool store often means 80-100+ ppm — they just don't want to tell you the fix is a partial drain, because draining doesn't sell products.

How to Fix High CYA

There is only one real fix: dilution. You must remove water containing high CYA and replace it with fresh water that has zero CYA.

No chemical lowers CYA. Despite what the pool store sells you, there is no product that reliably removes cyanuric acid from pool water. Some products claim to reduce CYA through biodegradation, but real-world results are inconsistent and unreliable. The only guaranteed method is dilution.

How to dilute:

Option 1: Partial drain and refill. Drain 1/3 to 1/2 of the pool and refill with fresh tap water.

Example: Your CYA is at 120 ppm. You drain half the pool and refill with tap water (0 ppm CYA). New CYA level: approximately 60 ppm (120 × 0.5 = 60). That's back in the acceptable range.

For a 15,000-gallon pool, draining half means removing about 7,500 gallons and refilling. At Frisco's water rates, the water cost is roughly $30-50. Compare that to the $200+ you've spent on algaecide, phosphate remover, and extra tabs trying to fight a problem that couldn't be fixed with chemicals.

Option 2: Multiple smaller drains. If draining half at once isn't practical (hydrostatic pressure concerns in clay soil, time constraints), drain 1/4 of the pool, refill, wait a day, and repeat. Two 25% drains achieve roughly the same dilution as one 50% drain.

Important Frisco warning: Never fully drain a pool in Frisco without professional guidance. An empty pool shell in Frisco's expansive clay soil can float from hydrostatic pressure. Partial drains (up to 50%) are safe. Full drains require hydrostatic relief valves and should be done quickly.

How to Prevent CYA From Climbing Again

After you've drained and refilled to reset CYA, the next step is stopping the source — the trichlor tabs.

Switch to liquid chlorine (sodium hypochlorite) as your primary sanitizer. Liquid chlorine adds zero CYA to the pool. You still need CYA for UV protection, but you add it once at the beginning of the season (granular stabilizer to reach 30-40 ppm) and let it naturally deplete through the year. Liquid chlorine maintains the chlorine level without continuously adding more CYA.

If you want to keep using tabs (some homeowners prefer the convenience), reduce the tab count significantly and supplement with liquid chlorine. Using one tab per week instead of three, plus liquid chlorine for the remainder of your sanitizer needs, adds CYA at roughly 1/3 the rate.

If you have a salt system, you're already using liquid chlorine (the salt cell generates it). CYA buildup from tabs shouldn't be an issue — unless someone is also putting tabs in the skimmer or floater "for extra chlorine." If you have a salt system, don't use tabs at all. The cell provides the chlorine. Add granular stabilizer once per year if CYA drops below 30 ppm.

What the Pool Store Won't Tell You

Pool stores sell trichlor tabs by the bucket — 25-pound, 50-pound buckets that last months. Tabs are their highest-margin product, and they sell millions of dollars worth every summer across North Texas. If the pool store told every customer "stop using tabs and switch to liquid chlorine," they'd lose their most profitable product line.

Instead, they test your water, see CYA at 120 ppm, and sell you algaecide ($15), phosphate remover ($25), and clarifier ($12) to treat the symptoms of a CYA problem they could solve with one sentence: "drain half your pool and switch to liquid chlorine."

We're not anti-tab — they have a place. They're convenient for vacation homes, for homeowners who can't add liquid chlorine regularly, and for pools with low CYA that need both chlorine and stabilizer simultaneously. But for a pool that's been on tabs for 2+ years, the CYA is almost certainly too high, and continuing to add tabs makes it worse every week.

The Reset Protocol

If your CYA is above 70 ppm and you want to fix it properly:

Week 1: Test CYA. Calculate how much water to drain based on current level and target of 40 ppm. Drain and refill over 1-2 days.

Week 2: Test CYA again to confirm the dilution worked. If still above 60 ppm, do a second smaller drain.

After the drain: Add granular stabilizer to bring CYA to 30-40 ppm (if the drain brought it below 30). Switch primary sanitizer to liquid chlorine. Stop using trichlor tabs.

Ongoing: Test CYA every 3-4 months. With liquid chlorine as the primary sanitizer, CYA should stay stable at 30-40 ppm year-round with only minor additions of granular stabilizer as needed.

Your chlorine will work again. The difference is immediate and dramatic. A pool that's been fighting algae for months at CYA 120+ will suddenly hold chlorine, stay clear, and respond to treatment the way it's supposed to. The chlorine was never the problem — the CYA was blocking it.

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