What Is Trichlor? Stabilized Pool Chlorine Tablets
Quick answer: Tri-chlor (trichloroisocyanuric acid) is the stabilized chlorine tablet sold as 3-inch pucks, with approximately 90% available chlorine. Every tablet dissolves slowly to release FC, but also adds Cyanuric Acid: roughly 0.6 ppm CYA per 1 ppm FC added. The tablet pH is acidic at 2.8, so extended use gradually lowers pool pH. Once CYA reaches target, switch to liquid chlorine to prevent further CYA accumulation.
Trichlor tablets are the most convenient form of pool chlorine, but convenience comes with tradeoffs that matter for long-term pool chemistry management. When CYA accumulates past target, see our pool shock treatment guide for how to reset the water before switching to liquid chlorine. Used correctly, they keep a pool sanitized with minimal daily effort. Used without tracking CYA, they create a slow-motion chemistry problem that ends in a partial drain. Understanding both sides of trichlor is essential for anyone managing low chlorine in pool situations or setting up a new maintenance routine.
How Tri-Chlor Works
Tri-chlor tablets, at 90% available chlorine, add about 0.6 ppm of cyanuric acid for every 1 ppm of free chlorine introduced into the water, while also reducing your pool’s pH slightly.
The chemistry is compressed into a slow-dissolve format. A 3-inch, 8-oz puck dropped in a floater or inline chlorinator releases chlorine over 5-7 days rather than instantly. The slow dissolution rate is what makes trichlor useful for maintenance: one or two tablets per week handles FC for most residential pools without daily dosing.
As tri-chlor dissolves, it releases three things into the water:
First, ensure that the active sanitizer in your pool system is hypochlorous acid, the critical Free Chlorine component. Next, maintain sufficient cyanurate ions to stabilize it as cyanuric acid, which will prevent breakdown by sunlight. Then, be aware of the acidity level; the dissolving tablet registers a pH of 2.8 and will gradually lower your pool’s pH over time.
The pH drift from trichlor is real and cumulative. Pools running exclusively on trichlor tablets for several weeks need periodic pH correction upward, usually with soda ash (sodium carbonate). Per EPA chlorine disinfection guidance{:target=“_blank”}, chlorine’s sanitizing efficiency drops above pH 7.8, so allowing pH to drift low and then overcorrecting high creates a sanitizing gap.
Tri-chlor vs. other chlorines
Choosing the right chlorine product is a core pool stabilizer (cyanuric acid protector) decision, not just a shopping convenience. The options have meaningfully different chemistry profiles.
| Product | Available Chlorine | pH | Adds CYA? | Adds Calcium? | Best Use |
|---|---|---|---|---|---|
| Tri-chlor tablets | ~90% | 2.8 (acidic) | Yes (+0.6 ppm/ppm FC) | No | Maintenance dosing via floater |
| Dichlor granular | ~56% | ~6.9 (neutral) | Yes (+0.9 ppm/ppm FC) | No | Hot tubs, vinyl startup |
| Cal-Hypo 65% | 65% | 11-12 (basic) | No | Yes | Outdoor pool shocking |
| Liquid chlorine | 10-12.5% | 12-13 (basic) | No | No | Best all-around for outdoor pools |
Trichlor has the highest available chlorine percentage of any common pool product at 90%. That concentration is why tablets are physically small relative to their chlorine output. The tradeoff is the acidic pH and mandatory CYA addition. Liquid chlorine at 10-12.5% is much weaker per unit volume, but it adds nothing to the water besides chlorine and it’s the recommended primary sanitizer once CYA is established.
When Tri-Chlor Is the Right Choice
Trichlor is genuinely useful in specific circumstances. It’s not the best choice for all pools.
Low-maintenance pools where the owner can’t dose daily. If you’re not testing and dosing chlorine every day, a tablet in a floater provides baseline FC protection between visits. A single 8-oz puck in a 10,000-15,000 gallon pool maintains FC for approximately 5-7 days.
Travel weeks. Loading a floater with trichlor before a week-long absence keeps the pool from going green from chlorine deprivation alone. This is the single best use case for trichlor in an established pool.
New pool or fresh fill with low CYA. A pool starting with zero CYA needs stabilizer anyway. Trichlor tablets during startup bring both FC and CYA up together. Stop once CYA reaches 30-50 ppm and switch to liquid chlorine.
What to avoid: Using trichlor in a saltwater pool. The salt chlorine generator already provides FC continuously. Adding trichlor tablets raises CYA steadily without any benefit, eventually reaching levels where the SWG can’t keep up with the FC demand. Also avoid trichlor as the sole chlorine source in pools that already have CYA above 40-50 ppm, per CDC pool disinfection guidelines{:target=“_blank”}.
CYA buildup math
A single 8-oz tri-chlor tablet dissolved in a 10,000-gallon pool adds approximately 3-4 ppm of Cyanuric Acid. Consistent weekly tablet use over a 20-week swim season can add 30-50 ppm of CYA, requiring a partial drain to reset.
Working through the math for a typical residential pool:
- 10,000-gallon pool, 1 tablet per week, 20-week season
- Each tablet adds ~3-4 ppm CYA
- Total CYA added: 60-80 ppm across the season
If the pool started the season at CYA 30 ppm (typical after a drain or winter), it ends at 90-110 ppm. At CYA 100 ppm, the TroubleFreePool minimum FC target rises to 7-8 ppm. Maintaining that FC level requires more chlorine than a pool at CYA 40 ppm. The pool becomes harder to manage, not easier.
The TFP CYA/FC chart makes this concrete: at CYA 30 ppm, minimum FC is 2 ppm. At CYA 80 ppm, minimum FC is 6 ppm. Trichlor use that pushes CYA from 30 to 80 triples the baseline chlorine demand.
The correct strategy: use trichlor until CYA reaches 40-50 ppm, then switch to liquid chlorine or cal-hypo for ongoing maintenance. Test CYA every 4-6 weeks during the season.
Dosing hardware
The hardware you use with trichlor tablets affects how well they work and how safely the pool surface is protected.
Floating dispensers. A basic floater costs $10-$25 and holds 1-3 tablets. The tablet floats on the surface dissolving into the water. The risk: if the floater gets trapped in a corner, against steps, or lodged against a vinyl liner, the concentrated acidic water around the tablet bleaches and etches the surface. Check the floater position daily. Don’t allow it to park against any surface.
Inline chlorinators. A plumbed inline chlorinator (Hayward CL220, Pentair Rainbow 320, similar) mounts to the return line after the heater and controls dissolution rate via an adjustable bypass valve. These units run $100-$250 installed and provide much more consistent FC delivery than floaters. The tablets dissolve in a controlled chamber, and the chlorinated water enters the return flow rather than concentrating near the tablet directly.
Offline chlorinators. Similar chemistry to inline, but plumbed with bypass lines that make servicing easier. Better suited to pools where the equipment pad is difficult to access for regular tablet refills.
Never put trichlor tablets in the skimmer basket. This routes highly concentrated acidic chlorinated water directly through the pump and filter every time the pump runs. It corrodes metal components, degrades pump seals, and attacks filter internals. It’s also harder to regulate dosage since skimmer flow rate varies.
FAQ
Can I put tri-chlor tablets directly in the skimmer?
We recommend against it. Routing concentrated trichlor water through the pump and filter on every pump cycle corrodes metal components and degrades seals over time. We find an inline chlorinator ($100-$250 installed) pays for itself quickly in avoided seal and O-ring replacements. It also makes dosage harder to control. Use a floating dispenser or inline chlorinator to protect your equipment.
How long does a tri-chlor tablet last?
A 3-inch, 8-oz trichlor tablet maintains efficacy for 5 to 7 days in a standard residential pool of 10,000 to 20,000 gallons with an 8 to 12-hour daily pump operation. Warmer water hastens the breakdown, while cooler temperatures slow it down. In inline chlorinator systems with partially closed bypass valves, expect the tablet’s life span to extend to about 10 to 14 days due to altered circulation patterns.
Does tri-chlor work in cold water?
Yes, but it dissolves more slowly. Below 60°F water temperature, dissolution rates drop significantly. This means less FC is released per day. For pools being used in shoulder seasons (spring/fall), this can result in lower-than-expected FC even with tablets in the floater. Test FC more frequently during cold weather rather than assuming the tablet delivery rate is consistent with summer performance.