What Is the SLAM Process? Pool Algae Treatment
Quick answer: SLAM (Shock Level And Maintain) is the TroubleFreePool method for killing pool algae and eliminating Combined Chlorine. Raise Free Chlorine to 40% of your CYA level, then hold it there using FAS-DPD tests until all three exit criteria pass: CC at or below 0.5 ppm, Overnight Chlorine Loss at or below 1 ppm, and water visually crystal clear.
The SLAM process is the most reliable chlorine-based approach for clearing pool algae that the residential pool community has tested at scale. We recommend it as the first response to any green or cloudy pool that hasn’t responded to normal dosing. We find it reliably clears even severe blooms within 4-7 days when the prep checklist is followed. It’s not a single shock dose. It’s a sustained treatment that continues until the algae is dead. If you have green pool water or persistent cloudiness that won’t clear, SLAM is the right framework. For proper pool chemistry maintenance that prevents algae from returning, see our full guide.
SLAM vs. “shocking”
Most pool stores recommend “weekly shocking” as routine maintenance. That advice refers to raising Free Chlorine by 1-2 ppm above normal levels. At that dose, FC rarely reaches the threshold needed to kill established algae. The store recommendation exists to sell product, not to solve algae problems.
SLAM is different. Instead of a single dose, you raise FC to a specific level based on your CYA reading and hold it there continuously, testing every 1-2 hours during the day. The process takes days, not hours.
SLAM also specifies the chlorine product: liquid chlorine (sodium hypochlorite) only. Cal-hypo and puck chlorinators are switched off during the process because each adds unwanted byproducts (calcium hardness, stabilizer) that complicate a multi-day treatment. Per EPA chlorine disinfection guidance{:target=“_blank”}, liquid chlorine leaves no residue other than hypochlorite ions, making it the cleanest option for sustained high-dose applications.
Before you SLAM: prep checklist
Running SLAM without completing the prep checklist is one of the most common reasons it fails. Do this before adding any chlorine.
1. Test and adjust CYA to 30-70 ppm. CYA is your stabilizer level. Below 30 ppm, FC degrades too fast under sunlight for SLAM to work. Above 70 ppm, the chlorine demand becomes impractical. If your CYA is above 70 ppm, do a partial drain and refill before starting. CYA above 90 ppm makes SLAM impossible without first diluting the water.
2. Adjust pH to 7.2-7.5. Do this once before you start, then leave pH alone. Chlorine interferes with pH test readings at SLAM-level FC, so testing pH mid-SLAM gives inaccurate results. Adjust once, then don’t check again until FC drops below 10 ppm after SLAM is complete.
3. Clean and inspect your filter. Algae comes off pool surfaces and loads the filter fast. A dirty filter at the start means a clogged filter by day two. Know how to backwash your sand or DE filter, or how to rinse a cartridge.
4. Get a FAS-DPD test kit. This is mandatory, not optional. Standard DPD test strips and drop-count tests can’t read FC accurately above 5-10 ppm. SLAM targets of 16-28 ppm are outside their range. The FAS-DPD method (using the R-0870/R-0871 reagents in a Taylor or TF-100 kit) reads accurately at all SLAM levels. Per CDC residential pool disinfection guidelines{:target=“_blank”}, accurate test equipment is the foundation of any effective chlorine treatment program.
Running the SLAM
Once prep is complete, follow these steps throughout the process:
1. Calculate your SLAM FC target. Multiply CYA by 0.4. A pool with CYA at 40 ppm runs SLAM at 16 ppm FC. CYA at 50 ppm requires FC at 20 ppm. Use the PoolMath app to calculate exact chemical additions for your pool volume.
2. Dose liquid chlorine to reach your target. For a pool under 20,000 gallons, plan to have 10 gallons of 10-12.5% sodium hypochlorite on hand to start.
3. Test and re-dose every 1-2 hours during daylight. At the start of SLAM, FC drops fast because active algae consumes it heavily. As algae dies, the consumption rate slows and re-dose intervals can stretch to 2-3 hours.
4. Brush the entire pool at least twice per day. Brush walls, floor, steps, behind ladders, inside skimmer walls, and under drain covers. Brushing puts dead algae cells into suspension where the filter can capture them.
5. Run the pump 24/7 for the entire SLAM duration. No exceptions. Stagnant water lets dead algae settle and active algae colonize undisturbed areas.
6. Backwash or clean the filter when pressure rises 25% above baseline. DE filters clear the fastest. Sand filters are the slowest. If you have a sand filter, adding pool-grade DE powder to the sand bed speeds clearing significantly.
Three SLAM-complete criteria
SLAM (Shock Level And Maintain) is completed only when all three TFP exit criteria pass simultaneously: Combined Chlorine under 0.5 ppm, Overnight Chlorine Loss under 1 ppm, and the water visually crystal-clear.
Worth doing.
Worth doing.
Worth knowing.
Test each criterion before declaring the SLAM done. All three must pass in the same testing window:
Worth knowing.
1. Combined Chlorine at or below 0.5 ppm. CC above 0.5 ppm means chloramines or organic contaminants are still being oxidized. SLAM continues.
2. Overnight Chlorine Loss Test (OCLT) at or below 1 ppm. Take an FC reading after sunset, then take another reading before sunrise (before UV hits the water). The difference must be 1 ppm or less. OCLT above 1 ppm means algae is still consuming chlorine overnight.
3. Water visually crystal clear. You must be able to see the pool floor in the deepest section with no haze. Cloudy blue water means dead algae particles are still in suspension. Continue filtering and wait.
If any one criterion fails, run another day of SLAM and test again the following evening and morning.
Typical SLAM duration
The SLAM chlorine target is 40% of the Cyanuric Acid level. A pool with CYA 40 ppm holds FC at 16 ppm until the algae clears. FAS-DPD testing is required because no other home test reads accurately at SLAM-level chlorine.
How long that takes depends on the severity of the bloom:
Common mistake.
- Light green (just turned green): 2-4 days for most residential pools
- Dark green or swampy water: 4-7 days, sometimes longer with slow sand filters
- Yellow (mustard) algae: 1-2 weeks; mustard algae is chlorine-resistant and may require a repeat SLAM after equipment is cleaned
- Black algae: 1-2 weeks minimum; black algae roots into concrete surfaces and requires aggressive manual scrubbing alongside SLAM
The filter type matters. DE filters clear dead algae the fastest. Cartridge filters run second. Sand filters can take a week or more to clear even after all the algae is dead. A sand filter owner running SLAM for the first time should add DE powder to the sand bed to improve filtration.
When SLAM Isn’t the Right Answer
SLAM is the right tool for active algae or elevated CC. It’s not the right tool for everything.
Cloudy water with stable FC and no green tint likely points to calcium scaling or high TDS rather than algae. Check Calcium Hardness and the Langelier Saturation Index before starting a SLAM that won’t solve the problem.
Small pool with easy drain-and-refill access. For a 5,000-gallon above-ground pool, the cost and time of SLAM can exceed the cost of draining and refilling with fresh water.
CYA above 100 ppm. You can’t run an effective SLAM at this CYA level because the required FC target becomes unsafe. The SLAM FC target would be 40+ ppm, which is unusable in practice. Drain 30-40% of the water, refill, retest CYA, and bring it under 70 ppm before starting.
For preventive strategies that keep algae from coming back, a quality pool algaecide applied after SLAM provides a secondary defense layer. Keeping pool water balance within range after SLAM is the most reliable way to prevent a repeat bloom. The bigger win, though, is daily FC testing and maintaining the correct FC/CYA ratio so SLAM is never needed again.
FAQ
Can I swim during SLAM?
Yes, with two conditions: you must be able to see the bottom of the deep end clearly, and FC must be between the minimum target and the SLAM level. At SLAM-level chlorine, the water is safe for swimmers despite the high FC. If you can’t see the pool floor, don’t swim.
How much does a SLAM cost in chlorine?
For a 20,000-gallon pool with a moderate green bloom, plan for 10-15 gallons of liquid chlorine across 3-5 days. At roughly $3-5 per gallon for 10-12.5% sodium hypochlorite, the chlorine cost runs $30-$75. Severe blooms requiring 7+ days can run $75-$150 in liquid chlorine.
Does SLAM damage pool plaster?
Running FC at SLAM levels (typically 16-28 ppm for most pools) for a few days doesn’t damage plaster, vinyl liners, or fiberglass. Extended exposure to very high FC over weeks would be another matter. But a standard SLAM duration doesn’t harm pool surfaces.
What if my CYA is too high to SLAM?
CYA above 90-100 ppm renders SLAM impractical. Drain 35% of water, replenish with fresh, then retest until CYA drops to 30-70 ppm before proceeding. No chemical can remove CYA from pool water, dilution is the sole remedy.