The SLAM pool method, explained
SLAM (Shock, Level And Maintain) is a community method for clearing algae: hold chlorine at a high level, day and night, until the water passes three end tests. Its levels are often above the swim limit on pool chlorine labels, so Pooldex doesn't calculate them. Below: what SLAM means, and a label-based way to use the same end tests.
Always follow your product’s label directions. Pool and spa chlorine, bromine, shock and algaecides are EPA-registered pesticides: use products labeled for pool or spa use, and don’t swim or soak until levels are within the limit on your product’s label. If this page and a label disagree, the label wins.
On this page (8 sections)
The calculator checks one of those end tests, combined chlorine. When it is 0.5 ppm or more, it scales the shock amount on your product's label to your gallons.
What SLAM means
SLAM stands for Shock, Level And Maintain. The name comes from the Trouble Free Pool community (we could only read their method through secondary sources). The idea behind it is chemistry the CDC also describes: the Model Aquatic Health Code explains that stabilizer (CYA) binds most free chlorine, so the more CYA a pool has, the more free chlorine it takes to do the same work. SLAM therefore sets its chlorine level from CYA rather than from a bag count.
SLAM differs from a one-time shock in one word: maintain. A single dose kills some algae and then fades. SLAM keeps chlorine high with repeated doses until nothing is left that uses it up.
Why we don't give SLAM numbers
Pool chlorine is a pesticide registered with the U.S. EPA, and its label is the legal limit for how you use it: the amount, how often, and when people can swim again. Many pool chlorine labels say not to swim above 4 ppm; some say 3 ppm. The CDC's model code annex notes that EPA has not approved pool products above 4 ppm with swimmers present. SLAM levels are usually well above that, and they can be above the treatment amount on some labels too.
So on this site and in the Pooldex app, treatment doses come from your product's label, scaled to your gallons. We don't publish a SLAM level or a SLAM dose. If you choose to follow a method that goes beyond your label, that is outside what Pooldex covers.
A label-based way to clear algae
- Test free chlorine (FC), total chlorine, pH, and CYA. A drop test (FAS-DPD) reads more precisely than most strips.
- Lower pH to 7.2–7.5 before you start, so chlorine works better. Wait at least 30 minutes after acid, with the pump running, before you add chlorine.
- Clean the filter, run the pump around the clock, and brush the whole pool.
- Treat with a chlorine product labeled for algae control, at the amount its label gives for algae, scaled to your gallons. Add it at dusk. Don't go above the label amount.
- Follow the label for any repeat dose. Test FC morning and evening. If the label allows a repeat treatment or a maintenance dose, use exactly that.
- Brush daily and clean the filter whenever pressure climbs.
- Run the end tests each night once the water looks clear (next section).
- Go back to your everyday target when all three pass. See pool chlorine levels.
Why liquid chlorine or cal-hypo? Dichlor and trichlor add stabilizer with every dose (0.9 and 0.6 ppm CYA per 1 ppm FC, according to PHTA), and higher CYA makes algae harder to clear at label levels. Cal-hypo adds calcium instead. See the shock guide for how products compare.
The 3 end tests: overnight loss, CC, clear water
These tests come from the SLAM method, but they are just measurements, and they work at any chlorine level.
| Test | How | Pass |
|---|---|---|
| Overnight chlorine loss (OCLT) | FC after sunset, FC again before sunrise, pool uncovered and unused | drop of 1 ppm or less |
| Combined chlorine (CC) | total chlorine minus free chlorine (the calculator above) | 0.5 ppm or less |
| Clear water | look at the main drain from the deck | drain clearly visible |
The overnight test tells you the most. Without sunlight, chlorine should barely drop overnight. A larger drop means something is still using it up: algae in the filter, plumbing, under ladder treads, or behind lights. Keep brushing and filtering, follow your label, and repeat the test the next night. If losses stay high while the water looks clear, read why a pool won't hold chlorine.
How long it takes
Clearing algae ends when the tests pass, not after a set number of days. We have not measured typical durations and don't publish a number. What makes it take longer: letting chlorine run out between doses, a dirty filter, low pump runtime, and high CYA.
When CYA is too high
The higher your CYA, the harder algae is to clear at label levels. Above about 50 ppm, even the everyday minimum for your CYA rises past the 4 ppm swim limit on many labels (see chlorine lock). Stabilizer only leaves with water: replacing half the water takes CYA from 80 to 40 when your fill water has none. The guide to lowering CYA shows how to plan a partial drain. Salt water pools often run CYA 60–80 on the cell maker's advice; check your cell's manual about algae and shocking. Hot tubs are a different case: follow your product label. For the full walkthrough, see the green pool plan.
Safety
- No swimming during algae treatment. Wait until the water is clear, the label's re-entry directions are met, and FC is at or below the swim limit on your label.
- Never mix chlorine with acid or with another chlorine product. Add them separately, with the pump running, at least 30 minutes apart.
- Wear gloves and eye protection when pouring; add chemicals to water, not water to chemicals. Store chlorine and acid apart, out of reach of children.
- In an emergency, call 911 or Poison Control at 1-800-222-1222.
- Always follow the product label. It is a violation of federal law to use a pesticide product in a manner inconsistent with its labeling.
FAQ
What does it mean to SLAM your pool?
It means raising free chlorine to a high level set by your CYA and holding it there with repeated doses until the water is clear, combined chlorine is 0.5 ppm or less, and overnight chlorine loss is 1 ppm or less. Those levels are often above the swim limit on pool chlorine labels, so we don't calculate them. Follow your product's label for treatment.
How long does SLAM take?
Until the three end tests pass. A lightly hazy pool can finish quickly; a dark green one takes longer. Steady brushing, a clean filter, and moderate CYA shorten it.
Can I swim during SLAM?
No. Chlorine during treatment can be far above the swim limit on pool labels, and cloudy water hides swimmers. Swim only once the water is clear and FC is at or below the swim limit on your label.
Sources
- CDC, Model Aquatic Health Code 2024, Annex, section 5.7.3.1.1.2 (chlorine and CYA; EPA label limit with swimmers present): cdc.gov (PDF)
- U.S. EPA, examples of pool chlorine labels with a re-entry limit of 4 ppm and 3 ppm: EPA label (PDF), EPA label (PDF)
- PHTA fact sheet, Dichlor: phta.org
- Hayward, AquaRite manual (CYA for salt cells): PDF
- Secondary: summary of the Trouble Free Pool FC/CYA method: poolchemtracker.com