Chlorine tablets dissolve slowly in water to kill bacteria and algae

Chlorine tablets are solid forms of chlorine that release the chemical gradually as they dissolve. They work by breaking down in your pool or spa water over several days, continuously sanitizing the water instead of requiring daily dosing like liquid chlorine. The two common types are trichlor (which also lowers pH) and dichlor (which raises pH slightly), so which one you use depends on your water's current chemistry.

Tablets are popular because they require less frequent attention than liquid chlorine and cost less per unit of sanitizer. They do require a delivery method — either a floating dispenser, an in-line feeder attached to your pump, or a skimmer basket — so the tablet sits in the right place and dissolves at the right speed.

Key Takeaways

  • Chlorine tablets must be placed in a dispenser or feeder, never directly in the pool or spa, because they can damage the surface and create unsafe chemical pockets.
  • A floating dispenser works for pools without a pump; an in-line feeder works best if your pump runs regularly and you want consistent dosing.
  • Test your water chlorine level twice a week and adjust the number of tablets or the dispenser opening to keep levels between 1 and 3 parts per million.
  • Tablets dissolve faster in warm water and slower in cold water, so you may need to add or remove tablets as the season changes.
  • Never mix chlorine tablets with other chemicals in the same container, and store them in a cool, dry place away from sunlight and moisture.

Choose a dispenser method that fits your setup

A floating dispenser is a plastic cage or basket that holds tablets and floats on the surface of your pool. Water enters through small openings, dissolves the tablet, and the chlorine disperses into the pool. Floating dispensers work well for above-ground pools and small spas, and they cost between $15 and $40. The downside is that they move around with the current and can get stuck against the wall, which slows dissolution in that spot.

An in-line feeder (also called a chlorinator) attaches to the return line of your pump system, so tablets dissolve inside the feeder as water flows through. This method gives you more control over how fast the tablets dissolve — you can adjust a dial to open or close the flow — and it keeps the tablets out of sight. In-line feeders cost $100 to $300 and work best if your pump runs at least 8 hours a day. They require basic plumbing knowledge to install, though many pool owners hire a technician.

A skimmer basket method means placing a tablet directly in your skimmer basket (the basket that catches leaves and debris). This is the cheapest option and requires no extra equipment, but it only works if your pump runs regularly, and the tablet can clog the basket as it dissolves.

Load and position tablets correctly

Start with one tablet per 10,000 gallons of water per week as a baseline. If your pool is 20,000 gallons, start with two tablets. Place the tablet or tablets in your chosen dispenser — do not break them apart — and close the dispenser securely so it does not come open in the water.

For a floating dispenser, set it in the pool away from the skimmer and pump intake, ideally in a spot where water movement will carry the dissolved chlorine around the pool. For an in-line feeder, load the tablets into the chamber according to the feeder's instructions, then set the dial to a medium opening. For a skimmer basket, drop the tablet in and let your pump do the work.

Check the dispenser or feeder every few days. If the tablet is nearly gone, add a new one before it dissolves completely. If the tablet is dissolving too slowly (still mostly intact after a week), you may need to increase water flow or switch to a faster-dissolving dispenser type.

Test and adjust chlorine levels twice weekly

Use a test kit or test strips to measure free chlorine at least twice a week. Test strips cost $10 to $20 for a bottle of 50 and give results in seconds — dip the strip in the water, wait the time shown on the bottle (usually 15 to 30 seconds), and compare the color to the chart. A liquid test kit costs $20 to $40 and is more accurate, but takes a few minutes and requires mixing chemicals.

Your target is 1 to 3 parts per million (ppm) for pools and 2 to 4 ppm for spas. If your level is below 1 ppm, add another tablet or increase the feeder opening. If it is above 3 ppm, remove a tablet or decrease the feeder opening. Wait 24 hours after any change before testing again, because the new tablet needs time to dissolve and distribute.

Test more often during hot weather, after heavy use, or after rain, because these conditions consume chlorine faster or dilute it. In winter, when the pool is less used and water is colder, you may need fewer tablets.

Adjust for seasonal changes and water temperature

Chlorine tablets dissolve much faster in warm water than in cold water. In summer, when water temperature is 80°F or higher, a tablet may dissolve in 3 to 5 days. In spring or fall, at 60°F to 70°F, the same tablet may take 7 to 10 days. In winter, below 50°F, tablets dissolve very slowly and may last two weeks or longer.

As the season changes, adjust the number of tablets you use. In early summer, you may need to add tablets more often. By late fall, you may need to remove tablets or switch to a lower-dose method. If you close your pool for winter, remove all tablets and store them properly so they do not degrade.

Heavy rain also dilutes chlorine, so test your water after a storm and add a tablet if the level has dropped below 1 ppm.

Store tablets safely and handle them correctly

Store chlorine tablets in their original container in a cool, dry place away from direct sunlight, heat, and moisture. A garage, shed, or basement works well. Do not store them near other chemicals, especially acids or algaecides, because chlorine can react with them and create toxic fumes or fire hazards.

Keep the container tightly sealed. Chlorine tablets absorb moisture from the air and lose strength if exposed, so an open container will degrade the tablets and waste your money. If you notice tablets clumping or turning yellow, they have absorbed moisture and should be replaced.

Wear gloves when handling tablets to avoid skin irritation, and wash your hands after. Do not breathe the dust from a broken tablet. If a tablet breaks, sweep up the pieces and dispose of them according to your local hazardous waste rules — do not pour them down the drain or throw them in the trash.

Troubleshoot common problems

If your chlorine level stays too high no matter how few tablets you use, your pool may be too small for tablet dosing, or the dispenser may be positioned in a dead spot where water does not circulate well. Try moving a floating dispenser to a different location, or switch to liquid chlorine that you can dose more precisely.

If your chlorine level drops very fast (below 1 ppm in two days), your pool may have high demand due to algae, heavy use, or very warm water. Test for algae using a test kit, and if present, shock the pool with a large dose of chlorine before resuming tablet use. If the pool is straightforward very warm or heavily used, add a second tablet.

If tablets are not dissolving at all, check that water is flowing through the dispenser or feeder. For a floating dispenser, make sure it is not stuck against the wall. For an in-line feeder, check that the pump is running and the dial is open. For a skimmer basket, verify the pump is on.

Frequently Asked Questions

Can I put chlorine tablets directly in my pool without a dispenser?

No. Tablets release chlorine too fast when placed directly on the pool floor or surface, which can bleach and damage the pool lining or concrete. They can also create pockets of very high chlorine concentration that are unsafe. Always use a dispenser, feeder, or skimmer basket.

How often do I need to add new tablets?

It depends on your pool size, water temperature, and usage. Most pools need a new tablet every 3 to 7 days in summer and every 7 to 14 days in cooler months. Check your dispenser every few days and add a tablet when the current one is nearly dissolved.

What is the difference between trichlor and dichlor tablets?

Trichlor tablets lower your pool's pH as they dissolve, which is useful if your pH is high. Dichlor tablets raise pH slightly. Test your water chemistry before choosing; if your pH is already low, use dichlor. If it is high, use trichlor.

Can I use chlorine tablets in a hot tub or spa?

Yes, but spas need higher chlorine levels (2 to 4 ppm) and the water is hotter, so tablets dissolve faster. Use a smaller dispenser or feeder designed for spas, and test the water more often — ideally daily — because spas have less water volume and chlorine is consumed faster.

What should I do if I accidentally overdose my pool with chlorine?

Stop adding tablets when ready and remove any dispensers or feeders from the water. Test the level daily. High chlorine will naturally decrease over time as it breaks down in sunlight, but you can speed this up by running the pump and leaving the pool uncovered. Do not enter the pool until the level drops below 5 ppm.