Pool-chemical safety: Read the whole label before opening the bucket. The Clorox trichlor labels say not to mix these tablets with other chemicals, never to add water to the product, and never to put them in a feeder or floater holding remnants of another product, because that can cause fire or explosion. CDC’s chemical-use poster says never to mix chlorine products with acid or with different chlorine products.
The In The Swim 3 Inch Stabilized Chlorine Tablets are the best 3-inch chlorine tablets for most pools that use a floater or automatic feeder. In The Swim lists them as 99% trichloro-s-triazinetrione with 90% available chlorine, with no added algaecide, and each tablet comes individually wrapped.
Choose Clorox Active99 if you dose through a plastic skimmer, HTH Advanced if algae keeps coming back, and Clorox XtraBlue if metal stains or scale worry you more. All four are stabilized trichlor, so all four add cyanuric acid (CYA) as they dissolve. If your stabilizer is already high, none of them is the right buy; see when a trichlor tablet is the wrong buy .
| Tablet | Active ingredient (label) | Added ingredients and claims | Where the maker allows it | Sizes and wrapping |
|---|---|---|---|---|
| In The Swim 3" | 99% trichlor; 90% available chlorine | None listed; maker says no fillers | Floater or automatic feeder; not the skimmer | 5, 10, 25, 50 lb; individually wrapped |
| Clorox Active99 3" | 99.0% trichlor; 90% available chlorine | No other active ingredient | Feeder, floater, or plastic skimmer | 35 lb |
| HTH Advanced 3" Wrapped | 93.6% trichlor plus 3.5% zinc sulfate monohydrate | Clarifier and algaecide | Feeder or skimmer; pump 8+ hours a day in a skimmer | 5, 8, 25 lb; individually wrapped |
| Clorox XtraBlue 3" | 94.05% trichlor; 84.65% available chlorine | Algae, corrosion, scale and stain claims | Feeder, floater, or plastic skimmer | 5, 12, 25, 35, 40 lb |
These are the makers’ published figures and EPA label data, not test results. The Clorox percentages come from the EPA-accepted master labels for the registrations EPA lists for each product, 67262-17 for Active99 and 67262-33 for XtraBlue. HTH’s figures come from the ingredient panel on its product page.
What every 3-inch trichlor tablet has in common
They add stabilizer. The Clorox Active99 label says the product “contains both chlorine and a stabilizer which reduces chlorine loss from sunlight.” CDC’s 2024 Model Aquatic Health Code (MAHC) annex explains that cyanuric acid combines with chlorine to form trichlor and dichlor, so CYA rises as the tablets dissolve.
Stabilizer changes the chlorine target. CDC’s home pool guidance recommends pH 7.0–7.8 and at least 1 ppm free chlorine in pools. If you use cyanuric acid or products such as trichlor, CDC recommends at least 2 ppm. For public pools, the MAHC caps CYA at 90 ppm. Home pools are not bound by that code, but it is a published public-health ceiling.
They are acidic. In The Swim notes that stabilized chlorine tablets “are acidic and may gradually lower pH over time,” so a tablet routine also needs regular pH tests.
They never go loose in the pool. Every label checked for this page says not to throw tablets directly into the pool, because they can damage surfaces.
1. In The Swim 3 Inch Stabilized Chlorine Tablets: best plain trichlor for floaters and feeders
Identity check: In The Swim sells these as “3 Inch Chlorine Tablets” in 5, 10, 25 and 50 lb buckets. The linked 25 lb bucket is the one the site’s trichlor versus dichlor comparison already uses.
Why it leads: the specification is simple. In The Swim lists 99% trichloro-s-triazinetrione and 90% available chlorine, describes the tablets as “non-blended, no fillers,” and wraps each tablet to limit moisture and skin contact. You get chlorine and stabilizer and nothing else, so it is easy to tell what any water change came from.
The compromise: In The Swim says not to put these tablets directly in the pool or the skimmer, “as damage will occur.” Plan on a floater or automatic feeder. Its product page also gives two different weekly starting amounts in two places, so follow the label on the bucket you receive.
Buy it if: you already run a floater or an inline or off-line feeder that permits trichlor.
Skip it if: the skimmer basket is your only dosing point.
2. Clorox Pool&Spa Active99 3-Inch Chlorinating Tablets: best plain trichlor for a plastic skimmer
Identity check: Clorox’s product page calls this the 35 lb “Chlorinating Tablets” with “99% Trichlor.” EPA lists Clorox Pool&Spa Active99 3-inch tablets under registration 67262-17. Its master label shows 99.0% trichloro-s-triazinetrione and 90% available chlorine.
Why it fits: it is chemically the same kind of plain trichlor as the In The Swim tablets. Unlike In The Swim, though, Clorox gives a skimmer direction: “add 1 tablet per 10,000 gallons to bring chlorine to 1 – 4 ppm.”
The compromise: the label sets conditions. It allows skimmer use “only if skimmer and the piping between skimmer and pump are plastic,” and warns that in vinyl pools, leaving the pump off for more than 8 hours with tablets in the skimmer can bleach and damage the liner. Clorox lists only a 35 lb size, which is a lot of oxidizer to store; check the tablet storage guide first.
Buy it if: your skimmer and piping are plastic and the pump runs daily.
Skip it if: you need a small container or your plumbing includes metal between skimmer and pump.
3. HTH Pool Care 3" Wrapped Chlorine Tabs Advanced: best with a built-in algaecide
Identity check: HTH sells the wrapped version as SKUs 42052W (5 lb), 42053W (8 lb) and 42055W (25 lb). The unwrapped Advanced line uses the same numbers without the W.
Why it fits: HTH’s ingredient panel lists 93.6% trichlor plus 3.5% zinc sulfate monohydrate. HTH describes the tablets as “3-in-1” (sanitizer, clarifier and algaecide) and says they are best for “pools with recurring algae issues or high phosphates.” Its directions say to add one tablet per 10,000 gallons every week, or as often as needed, to keep free chlorine at 1–4 ppm.
The compromise: the extra ingredients do not cancel the stabilizer. HTH itself says the Advanced tablets “contain approximately 57% cyanuric acid by weight.” In a skimmer, HTH says to run the pump at least eight hours a day and to start with a skimmer that is “completely clean and free of residue from other water treatment products.”
Buy it if: algae keeps returning between shock treatments and you want one tablet instead of a separate maintenance algaecide.
Skip it if: you prefer to add an algaecide separately, or only when the water needs it.
4. Clorox Pool&Spa XtraBlue 3-Inch Chlorinating Tablets: best for stain, scale and corrosion claims
Identity check: EPA lists Clorox Pool&Spa XtraBlue chlorinating tablets under registration 67262-33. Its master label shows 94.05% trichloro-s-triazinetrione and 84.65% available chlorine. Clorox sells 5, 12, 25, 35 and 40 lb sizes.
Why it fits: Clorox says XtraBlue “kills and prevents green, mustard and black pool algae” and “prevents corrosion, scale and stains.” That makes it the pick here for pools where metal staining or scale is the main worry. It also has a skimmer direction: “add 2 tablets per 10,000 gallons to bring chlorine to 1 – 4 ppm.”
The compromise: its available chlorine (84.65%) is lower than the plain 99% trichlor tablets (90%), because part of the tablet is other ingredients. The same plastic-skimmer and 8-hour vinyl limits from the Clorox label apply.
Buy it if: you want one tablet that claims stain and scale protection along with sanitizing.
Skip it if: you want the highest available chlorine per pound.
Read the label before the first tablet
Each maker states its own starting amount, and those amounts differ. Use the directions on the bucket you buy, then adjust from your free chlorine test, not from another brand’s number. The feeder dosing-rate check explains how to change one setting at a time.
The Clorox labels add these rules:
- “Do not add this product to any dispensing device containing remnants of any other product. Such use may cause a violent reaction leading to fire or explosion.”
- “Never add water to product. Always add product to large quantities of water.”
- “Reentry into treated swimming pools is prohibited above levels of 4 ppm chlorine due to risk of body injury.”
HTH’s directions add: “Do not use with any other tablets or sticks in the same skimmer, floater, or feeder,” and “Use only in a new feeder or device that has previously contained only this product or trichloro-s-triazinetrione.” The feeder fit map covers which feeders accept 3-inch trichlor at all.
When a trichlor tablet is the wrong buy
If CYA is already high, more trichlor raises it further. The MAHC annex describes partial draining and refilling as the way to lower CYA, and accepts other methods only when test data confirm the lower level. HTH says to switch to its HTH Brilliance Cal Hypo Chlorine Tablets (a labeled Amazon search) “when CYA approaches 50 ppm.” Those tablets add no cyanuric acid. They need their own floater, feeder or empty skimmer basket, though: HTH says never to use them in equipment that has held trichlor, and never to mix trichlor and cal hypo in the same device. Liquid chlorine is the other stabilizer-free route; the tablet versus liquid comparison weighs it.
How to choose
- Start with the dosing device. A floater or feeder accepts any of these four if its manual permits trichlor. A skimmer narrows the field to the three with skimmer directions, and only with plastic plumbing.
- Decide whether you want extras. The plain 99% trichlor tablets (In The Swim, Active99) list 90% available chlorine. XtraBlue lists 84.65%, and HTH does not publish a figure for Advanced; both add algae or stain claims instead.
- Check stabilizer first. Test CYA before buying a season’s supply. A tablet routine only works while CYA stays in range; the stabilizer guide lists the published ranges.
- Buy what you can store. Keep tablets dry, in the original container, and away from acids and other chlorine products.
Tablet diameter is a fit question, not a dose question. The 1-inch versus 3-inch comparison covers it, and the tablet choosing guide covers pool volume and testing schedules.
Sources
- In The Swim 3 Inch Chlorine Tablets, 25 lb and size listing
- Clorox Pool&Spa Chlorinating Tablets (Active99)
- Clorox Pool&Spa XtraBlue Chlorinating Tablets
- HTH Pool Care 3" Wrapped Chlorine Tabs Advanced and 3" Chlorine Tabs Advanced
- HTH Brilliance Cal Hypo Chlorine Tablets
- EPA product search: Clorox Pool&Spa registrations
- EPA-accepted label, registration 67262-17 (June 26, 2024)
- EPA-accepted label, registration 67262-33 (March 7, 2025)
- CDC: Home Pool and Hot Tub Water Treatment and Testing
- CDC: Use Pool Chemicals Safely poster
- CDC: 2024 Model Aquatic Health Code and annex
Product facts were checked against these sources on October 9, 2026. This is researched buyer analysis without hands-on testing; confirm the size and label on the listing before ordering.
FAQ
What are the best 3-inch chlorine tablets?
For a floater or automatic feeder, In The Swim’s 99% trichlor tablets are the plain choice. Clorox Active99 suits a plastic skimmer, HTH Advanced adds an algaecide and clarifier, and Clorox XtraBlue adds stain and scale claims.
Do 3-inch chlorine tablets raise stabilizer (CYA)?
Yes. Trichlor tablets contain both chlorine and stabilizer, so cyanuric acid builds up as they dissolve. Test it regularly, and switch to a chlorine source without stabilizer when it runs high.
Can I put 3-inch tablets in the skimmer?
Only if that product’s label allows it. In The Swim says no. Clorox’s label allows its tablets in a skimmer only if the skimmer and the piping to the pump are plastic, and HTH says to run the pump at least eight hours a day.
How many 3-inch tablets do I need?
Use the directions on the label of the bucket you buy, then adjust to your free chlorine test. Makers state different starting amounts, so a number from one brand does not carry over to another.