A green pool is an algae bloom — and while it looks dramatic, it's a solvable problem in most cases without draining the pool. The key is treating it in the right sequence. Skipping steps or treating in the wrong order wastes chemicals and time. Done correctly, most green pools can be cleared within 24–72 hours.
Before you start: assess the severity
The treatment approach depends on how bad the algae bloom is:
- Light green / teal tint: Water is slightly discolored but you can still see the bottom. This is early-stage algae. Treat aggressively and it will clear quickly.
- Green / murky: Visibility is significantly reduced. Standard treatment sequence applies — plan for 48–72 hours to clear.
- Dark green / swamp: You cannot see the bottom at all. This is a severe bloom. Treatment is the same sequence but will require more chemicals and more time. If the water is also black or has significant sludge on the bottom, a drain and acid wash may be more cost-effective than chemical remediation.
Step 1: Test the water
Before adding anything, test your water chemistry — specifically pH, alkalinity, and cyanuric acid. This matters because:
- Chlorine is far less effective above pH 7.8. If your pH is high, shocking the pool without adjusting pH first wastes most of the chlorine you add.
- High cyanuric acid (above 80–100 ppm) can neutralize chlorine to the point where shocking has almost no effect — a phenomenon called "chlorine lock." If CYA is very high, a partial drain and refill may be necessary before treatment will work.
Step 2: Adjust pH to 7.2
Lower pH toward 7.2 using muriatic acid or a pH decreaser before shocking. At pH 7.2, chlorine's sanitizing effectiveness is approximately double what it is at pH 7.8. This step significantly reduces the amount of shock needed and speeds up the clearing process.
Step 3: Brush every surface thoroughly
Brush walls, steps, ledges, and the floor before adding any shock. Algae clings to surfaces in a protective biofilm — brushing disrupts this and exposes the algae cells to the chlorine you're about to add. Skipping this step significantly reduces the effectiveness of the shock treatment.
Use a stiff nylon brush for plaster or pebble surfaces. Use a soft nylon brush for vinyl liners — stiff brushes can tear the liner.
Step 4: Shock the pool
Use calcium hypochlorite shock (granular, typically 65–73% available chlorine) — not stabilized chlorine tabs or dichlor. For a green pool treatment, you need to raise free chlorine to 10–30 ppm depending on severity:
- Light green: 10–15 ppm (approximately 2 lbs of cal-hypo per 10,000 gallons)
- Moderate green: 15–20 ppm (3 lbs per 10,000 gallons)
- Dark green: 20–30 ppm (4+ lbs per 10,000 gallons)
Add shock in the evening or at night — UV degrades chlorine rapidly and you'll lose significant effectiveness if you shock in direct afternoon sun. Dissolve granular shock in a bucket of water before adding to the pool to avoid bleaching the surface.
Step 5: Run the filter continuously
Run your pump and filter continuously — 24 hours a day — throughout the treatment process. Dead algae cells need to be filtered out. A cartridge filter will need to be cleaned more frequently than usual during this process (check and rinse every 12–24 hours). A DE filter will need to be backwashed. A sand filter may need backwashing every 8–12 hours during heavy clearing.
Step 6: Add algaecide (optional but helpful)
A quality algaecide added after the initial shock treatment helps prevent algae from re-establishing. Use a polyquat-based algaecide rather than a copper-based one if you have a plaster, pebble, or colored pool surface — copper can stain. Follow the label dosing instructions based on your pool volume.
Step 7: Watch the color change
As treatment works, your pool will typically go through a color progression: green → cloudy blue-grey → clearing. The blue-grey stage means the algae is dead but hasn't been fully filtered out yet — this is normal and means the treatment is working. Keep the filter running and test chlorine levels every 12–24 hours to confirm they're staying elevated.
Step 8: Vacuum to waste
Once the water begins clearing, vacuum the pool floor to waste (bypassing the filter) to remove the settled dead algae. Vacuuming through the filter at this stage will clog it rapidly. Set your multiport valve to "waste" and vacuum slowly — you'll lose water level doing this, so have a hose running to maintain the water level above the skimmer.
When to call a professional
If the pool hasn't begun clearing within 48 hours of treatment, or if you've shocked multiple times without improvement, it's time to have a professional assess the situation. Persistent green pools often have underlying issues — very high CYA, a failing filter, or phosphate levels that are overwhelming the chlorine — that need diagnosis before more chemicals will help.