How Green Is Green?
Before treating, assess the severity. Treatment intensity depends on how far gone the pool is:
| Level | Description | Recovery Time | Shock Level |
|---|---|---|---|
| Light green | Can still see bottom, green tint | 24 hours | 10–15 ppm FC |
| Medium green | Can't see bottom, solid green | 48 hours | 20–25 ppm FC |
| Dark green/swamp | Opaque, can't see 6 inches deep | 72+ hours | 30+ ppm FC |
The SLAM Method
SLAM stands for Shock, Level, And Maintain. It's the proven method pool professionals use to clear green pools:
- Shock to the correct FC level for your CYA
- Level — maintain that FC level continuously (don't let it drop)
- And Maintain — keep brushing, filtering, and testing until clear
The key insight: you must maintain the elevated chlorine level, not just add one big dose and hope. If FC drops overnight, the algae recovers.
Step-by-Step Recovery
Step 1: Test the Water
Test for FC, CC, pH, CYA, and TA. You need CYA to calculate the correct shock level.
Target FC by CYA level:
| CYA | Target FC (SLAM level) |
|---|---|
| 0 | 10 ppm |
| 30 | 12 ppm |
| 50 | 20 ppm |
| 70 | 28 ppm |
| 100 | 39 ppm |
Step 2: Adjust pH First
Lower pH to 7.2 before shocking. Chlorine is much more effective at lower pH. Use muriatic acid.
Step 3: Brush EVERYTHING
Before adding chlorine, brush all surfaces — walls, floor, steps, behind ladders, inside skimmer throats. Every surface.
Step 4: Add Liquid Chlorine
Use liquid chlorine (sodium hypochlorite 10–12.5%) for green pool recovery. Don't use cal-hypo (adds calcium you don't need) or dichlor (adds CYA you definitely don't need).
Use our shock calculator for exact amounts.
Step 5: Run the Filter 24/7
The filter does the actual clearing. Run it continuously until the pool is clear.
- Clean/backwash the filter every 8–12 hours during recovery
- DE filters may need to be broken down and cleaned manually
- Cartridge filters may need replacement if heavily clogged
Step 6: Test and Maintain FC Every 4–6 Hours
This is where most people fail. You must maintain the SLAM FC level continuously. Test every few hours during the day and add more chlorine as needed.
Step 7: Vacuum to Waste
Once the water turns from green to cloudy gray/white, vacuum the dead algae to waste (bypass the filter). This is a huge time saver.
Step 8: Pass the Three Tests
You're done when:
- Water is clear — You can see the main drain
- CC is 0.5 ppm or less — Combined chlorine has been destroyed
- FC holds overnight — Lose less than 1 ppm FC in 8 hours
Timeline: What to Expect
| Time | What You'll See |
|---|---|
| 0–6 hours | No visible change (normal) |
| 6–12 hours | Green may darken slightly (algae dying) |
| 12–24 hours | Green shifts to blue-gray or cloudy white |
| 24–48 hours | Cloudiness clearing, can start to see bottom |
| 48–72 hours | Clear water (most pools) |
Preventing the Next Green Pool
- Maintain FC at 7.5% of CYA level
- Never let the pool go more than 7 days without testing
- Brush weekly even when water looks perfect
- Track chemistry trends — PoolOps shows you when readings are drifting before they become problems