SLAM (algae clear-out) is an operating mode: set a shock-level FC target based on your CYA, then hold FC at that target while brushing and filtering until the pool is genuinely stable.
If CYA is high, “normal FC” can be too weak to stop algae. Random shocks create spikes, then FC crashes and algae regains ground.
Why “shock once a week” doesn’t fix persistent green water (especially with high CYA)
Algae isn’t defeated by a single chlorine spike. It’s defeated by enough effective chlorine for long enough, plus removal of the material that keeps consuming chlorine (debris, biofilm, dead algae).
The usual failure loop
- CYA rises quietly (tabs/granular products), but daily FC targets don’t rise with it.
- Chlorine spikes after “shock”, then drops below effective strength for hours.
- Debris + biofilm keep demand high, so FC crashes faster than expected.
- Dead zones (steps, corners, behind lights) keep seeding the pool unless brushed.
Two “looks better” traps
- Blue-but-not-stable: water can change color before algae demand is gone.
- Clarifier illusion: clearer water doesn’t mean sanitiser level is correct.
Use end-criteria (Block 3) to confirm you’re finished.
Preparation: tests + mechanical clean-up
Start SLAM with clean baskets, decent circulation, and the key test numbers. Skipping prep usually means more chlorine and more days.
Test before you raise FC
- FC & CC (progress and stability tracking)
- pH (set first; high FC can distort many pH tests)
- CYA (your SLAM FC target depends on this)
If your testing is inconsistent, SLAM becomes guesswork. Reliable measurement is part of the protocol.
Mechanical steps that cut chlorine demand
- Remove debris (skim, net, vacuum). Less debris = less chlorine consumed.
- Brush everything: walls, floor, steps, ladders, skimmer throat, behind fittings.
- Empty baskets (skimmer + pump) and restore flow.
- Start filtration and plan longer run-time during clean-up.
What not to do
- Don’t rely on “shock products” that keep adding stabiliser if CYA is already high.
- Don’t skip brushing. Chemistry alone often leaves algae protected in biofilm and dead zones.
Set the SLAM FC target by CYA — then hold it
SLAM succeeds when FC stays near the target. Early on, chlorine demand can be high; later, it drops as organics and algae are removed. Use liquid chlorine for predictable FC increases without adding stabiliser.
A big dose once is less effective than smaller top-ups that keep FC from falling under the target. If FC keeps crashing, assume the pool is still consuming chlorine (organics/algae) and tighten the test/top-up rhythm.
Adjust pH before high FC. Once FC is very high, many pH tests become unreliable.
Table 1 — CYA band → SLAM FC target → notes
| CYA band (ppm) | SLAM FC target (ppm) | Notes |
|---|---|---|
| 20–30 | 10–12 | Lower CYA burns off faster in sun; expect more daytime top-ups. |
| 40 | 16 | Common outdoor band. Keep FC from drifting under target between doses. |
| 50 | 20 | A frequent “recurring algae” level if everyday FC is run too low. |
| 60 | 24 | If demand stays extreme, focus on brushing/removal and circulation, not bigger spikes. |
| 70 | 28 | Many salt pools sit here. Cell output alone often can’t hold SLAM FC; supplement. |
| 80 | 31 | High CYA pushes targets high; consider whether controlled dilution is more practical. |
| 90 | 35 | Testing accuracy matters a lot. If results are inconsistent, avoid guessing. |
| 100 | 39 | Often more practical to lower CYA first via a controlled partial drain/refill plan. |
Filtration and filter maintenance during SLAM
Chlorine kills algae; the filter removes what’s left. During SLAM the filter loads quickly, especially as dead algae turns into fine haze.
Filter handling rules
- Record a clean pressure baseline after a proper clean/backwash.
- Clean/backwash when pressure rises meaningfully from baseline (follow manufacturer guidance).
- Keep baskets clear to maintain flow and circulation.
- Run filtration longer than normal while the pool is clearing.
Backwashing and CYA (what to expect)
Backwashing removes water, so it can reduce CYA gradually — but it’s not predictable. If CYA is very high, a planned partial drain/refill is the controlled approach.
Table 2 — Daily checklist (test / brush / filter check)
| When | Action | Purpose |
|---|---|---|
| Morning | Test FC; dose back to SLAM target. Skim; check baskets. | Restores effective chlorine for the day and reduces demand from debris. |
| Midday | Re-test FC (if possible); top up. Brush steps/corners/dead zones. | Prevents long low-FC windows; brushing exposes algae and lifts fines. |
| Evening | Test FC; dose to target. Vacuum/robot; remove settled debris. | Sets up overnight stability and helps filtration catch up. |
| Daily | Check filter pressure vs baseline; clean/backwash when needed. | Maintains circulation and capture of dead algae. |
| As needed | Inspect hidden zones (behind lights, ladders, skimmer throat). | Stops re-seeding that makes SLAM feel “endless”. |
Bar chart — Typical chlorine demand drivers during cleanup
Where chlorine usually goes during SLAM (conceptual)
Use this to troubleshoot: reduce the driver (remove debris, improve circulation, clean filter) instead of only increasing dose.
Fallback summary:
- Organics/debris: often the biggest chlorine sink until removed.
- Active algae/biofilm: drops as brushing + FC hold break it down.
- Sun/UV: higher mid-day/summer; CYA moderates but doesn’t eliminate loss.
- Circulation gaps: dead zones protect algae unless brushed and flow is restored.
If SLAM feels “stuck”
- Cloudy but not green: keep holding target FC; you’re often filtering dead algae fines.
- Green returns quickly: suspect hidden zones, insufficient brushing, or long low-FC gaps.
- FC demand never improves: re-check debris removal, baskets, filter condition, and whether CYA is too high to be practical.
Finish criteria: clear + stable + ready to return to normal FC/CYA
Ending early is the main reason algae comes back. Use end-criteria that confirm both appearance and chemistry stability.
- Water is truly clear (not hazy; deep end details visible).
- CC stays low (commonly around ≤ 0.5 ppm, depending on test method).
- Overnight loss is minimal: test FC after sunset and again before sunrise; a small drop (often ~1 ppm or less) indicates algae demand is largely gone.
If overnight numbers are inconsistent, improve testing reliability before making conclusions.
Common “not finished yet” signs
- FC still crashes quickly even with good dosing rhythm.
- Cloudiness improves, then stalls while filter pressure climbs rapidly.
- Green tint returns in corners/steps within a day or two.
After SLAM: step down safely and lock in prevention
When SLAM is complete, let FC drift down from SLAM level to your normal FC/CYA operating range and keep prevention tight while the pool “settles.”
Post-SLAM routine (simple and effective)
- Maintain daily FC appropriate for your CYA (don’t let it fall into “low for this CYA” territory).
- Brush regularly for the next 1–2 weeks, especially steps, corners, and behind returns.
- Re-check pH once FC is back in a normal testable range, then rebalance if needed.
- Re-check CYA if you did significant backwashing or water replacement.
When to call Complete Pool Maintenance
- CYA is very high and SLAM targets are impractical; you need a controlled dilution plan plus follow-up balance.
- Filtration isn’t keeping up (persistent haze, suspicious pressure behavior, weak flow).
- Recurring algae despite correct targets and brushing (hidden zones or equipment issues often involved).
Equipment concerns: inspection & repair · Troubleshooting hub: Problems hub
FAQ
Can I SLAM with a salt chlorinator?
Yes, but most salt cells can’t raise and hold SLAM FC fast enough during the early high-demand phase. A practical approach is to keep the chlorinator running for baseline production and supplement with liquid chlorine to maintain the target.
Do I need to lower CYA before SLAM?
Not always. If CYA is moderate and you can test/dose reliably, SLAM at the correct target works well. If CYA is very high, targets climb and testing accuracy becomes critical; many owners choose controlled partial water replacement first to make SLAM practical.
Floc/clarifier — when is it appropriate?
These can help when algae is already dead and you’re dealing with fine haze that the filter struggles to capture. They are not a substitute for correct FC/CYA targeting and can create extra clean-up or filter issues if used too early.
- More appropriate: after FC demand has dropped and the pool is no longer actively green.
- Less appropriate: when FC is still crashing or green water persists (root cause not solved).
One-line checklist to avoid “endless shocking”
- Correct SLAM target by CYA
- Hold FC near target (avoid long low-FC gaps)
- Brush + remove debris + keep filtration effective
- Finish with clear end-criteria (clarity + CC low + minimal overnight loss)