Why Your Pool Turns Green After Opening & How to Fix It

Opening your pool for the season should feel exciting, not disappointing. A green pool is usually the result of algae growth, poor chemical balance, or leftover contaminants from winter. Understanding what causes it will help you avoid it next year and fix it fast now.

Algae Growth

Algae thrive in warm, untreated water. If your pool cover had leaks or the water sat stagnant for months without proper chemicals, algae spores likely took over.

Low Chlorine Levels

Chlorine is what keeps your pool water sanitized. After months of sitting unused, chlorine can dissipate. Without enough sanitizer, algae and bacteria quickly multiply.

Imbalanced Water Chemistry

When pH, alkalinity, or stabilizer levels fall out of range, chlorine becomes less effective. Imbalanced chemistry opens the door for green, murky water.

Organic Debris

Leaves, dirt, and other organic matter can decay over winter. This fuels algae and stains the water, especially if your cover was loose or damaged.

How to Fix a Green Pool Fast After Opening

Don’t panic if your pool water is green. Follow these steps to clear it up and prevent long-term problems.

Step 1: Remove Debris

• Skim the surface to remove leaves and floating debris.

• Use a leaf net or rake to scoop out the bottom.

• Avoid vacuuming at this stage, since it can stir up algae.

Step 2: Test Your Water

• Use a reliable test kit or take a sample to a pool store.

• Check chlorine, pH, alkalinity, stabilizer (CYA), and phosphate levels.

• Record results so you can rebalance everything correctly.

Step 3: Adjust pH and Alkalinity

• Get your pH between 7.2 and 7.4 to make chlorine more effective.

• Adjust total alkalinity to 80-120 ppm.

• Use pH increaser or decreaser and alkalinity buffers as needed.

Step 4: Shock the Pool

• Add a strong chlorine shock (calcium hypochlorite or sodium dichlor).

• Use at least 2–3 times the normal dose to fight severe algae.

• Broadcast it across the deep end while the pump is running.

• Run the filter for 24–48 hours non-stop.

Step 5: Brush All Pool Surfaces

• Use a stiff-bristled pool brush.

• Scrub walls, floor, steps, and ladders.

• Break up algae colonies so chlorine can reach them.

Step 6: Vacuum the Pool

• After letting debris settle, vacuum the pool floor.

• Use the waste setting if possible to bypass the filter.

• This removes dead algae and prevents clogs.

Step 7: Run and Clean the Filter

• Run the filter continuously until the water clears.

• Backwash sand or DE filters often.

• Clean cartridge filters thoroughly.

Step 8: Recheck and Rebalance Chemistry

• Retest after 24–48 hours.

• Adjust chlorine, pH, calcium, and CYA as needed.

• Keep chlorine between 1–3 ppm moving forward.

Preventing a Green Pool Next Opening

Fixing a green pool takes work, but preventing it is simple with consistent care.

During Closing Season

• Balance all chemicals before covering your pool.

• Add an algaecide and chlorine shock before winter.

• Use a high-quality, tightly sealed pool cover.

Throughout Winter

• Check the pool cover monthly.

• Remove standing water and debris.

• Re-check water chemistry if you live in a warmer climate.

Before Opening Again

• Remove the cover carefully to avoid spills.

• Start the filtration system and shock the pool immediately.

• Test and balance chemistry before the first swim.

Common Mistakes to Avoid

• Ignoring water balance: Skipping chemistry checks lets algae thrive.

• Under-shocking: Weak shock treatments don’t kill tough algae.

• Not brushing: Algae stick to surfaces and survive if not scrubbed.

• Waiting too long: Green water worsens fast without treatment.

When to Call a Professional

If the green won’t go away after shocking, or if you notice cloudy water with no improvement, it might be time for help. A pool service can diagnose stubborn problems and restore your pool faster.

You don’t have to let green water ruin your pool season. With the right steps and consistent maintenance, your pool will stay swim-ready and sparkling from the moment you open it.