Why Pool Water Is Green at Opening (and How to Fix Fast)

You pulled the cover off your pool in April and the water is solid green. Algae has been growing all winter and the pool looks like a swamp. The good news: a properly executed green-pool recovery gets you swimmable water in 5–7 days, not weeks. Here’s the fast version.

Step 1: Skim and brush before anything else

Skim leaves and floating debris. Then brush every surface of the pool aggressively — walls, floor, steps, light niche. Algae anchors to surfaces. If you don’t physically break up the colonies, even massive shock doses won’t kill them.

Step 2: Balance pH FIRST

Chlorine works dramatically better at correct pH. Test pH and adjust to 7.4–7.6 BEFORE shocking. Skipping this step is why most owners shock multiple times without making progress.

Step 3: Triple-shock (or quad)

For a 20,000 gallon pool with green water, you need 5–7 lb of cal-hypo to reach 15–20 ppm free chlorine. Pre-dissolve in buckets of water. Pour around the perimeter with the pump running. Repeat 12 hours later if water hasn’t turned from green to cloudy gray.

Step 4: Run pump 24/7

The pump must run continuously during recovery. The filter is doing the heavy lifting once chlorine kills the algae — pulling dead cells out of the water. Cleaning the filter daily during recovery is normal.

Step 5: Add clarifier (after shock clears)

Once water turns from green to cloudy gray, add polymer clarifier. This binds the microscopic dead algae cells so the filter can capture them.

Step 6: Vacuum to WASTE

If the pool floor is covered in dead algae sludge, vacuum to WASTE (multiport position) rather than FILTER. The sludge would overload the filter immediately. Top off water as you vacuum.

Recovery chemistry pack

Recovery timeline

• Day 1: Brush, balance pH, shock. Pool turns from green to cloudy gray within 24 hours.
• Day 2: Re-shock if still green. Brush again. Vacuum to waste if sludge accumulating on floor.
• Day 3–4: Water cloudy but lightening. Add clarifier. Filter pressure rising; clean it daily.
• Day 5–7: Water clears. Retest all parameters. Adjust calcium, alkalinity, stabilizer as needed.

For a recovery kit matched to your pool size, send PST Pool Supplies your pool gallons and the photo of the green pool.