The Short Answer: Green pool water is caused by algae growth that occurs when chlorine levels drop, pH balance rises outside the ideal range, or filtration and systems fail to keep water moving throughout the pool. Fixing a green pool requires balancing water chemistry, shocking with a high dose of chlorine, and running your filter continuously until the water clears.
You step outside ready to enjoy a refreshing swim, only to discover your swimming pool has a murky green tint. What went wrong? How did this happen? And, more importantly, how do you fix it?
A green pool is one of the most common pool problems homeowners face, and the good news is that it’s almost always fixable. Green pool water typically indicates algae growth, a natural occurrence when the pool isn’t being maintained or its chemistry falls out of balance.
Green pools are usually caused by:
- Chemical imbalances (including low chlorine levels)
- Filtration and circulation problems
- Environmental factors like heavy rain or heat waves.
Understanding Why Pools Turn Green
The Role of Algae
Algae are microscopic aquatic organisms that exist everywhere in the environment. When pools aren’t properly cleaned, these organisms can rapidly multiply, creating a green, cloudy appearance on the water.
Pool algae come in several varieties, including green algae (the most common type), yellow or mustard algae, and black algae (the most stubborn variety). Green algae thrives in the same warm, sunny conditions that make pools enjoyable for swimming.
The Chemistry Connection
Your pool’s chemical balance determines whether algae can survive and multiply or whether sanitizers keep them in check. Chlorine kills algae and bacteria, but only when present in the right quantities.
pH levels dramatically impact how well your sanitizer works. With very high pH levels, chlorine becomes less active, allowing algae to flourish. Maintaining balanced water chemistry between 7.0-7.8 pH prevents algae growth by allowing your sanitizer to work properly.
Common Causes of Green Pool Water
Low or Depleted Chlorine Levels
Chlorine serves as your pool’s primary defense against algae and bacteria. When free chlorine levels drop below the minimum range of 1 part per million (ppm), an algae outbreak could occur.
Several factors cause low chlorine levels:
- Lack of maintenance, especially during vacations and colder seasons
- Heavy pool usage, which consumes chlorine faster
- Heavy rainfall, which dilutes existing concentrations
Outdoor pools require more frequent chlorine additions than indoor pools due to UV exposure from warm weather.
Improper pH Balance
Pool pH should remain between 7.0 and 7.8 for the best water quality. When pH rises above this range, chlorine’s sanitizing power diminishes dramatically. This means even pools with adequate chlorine levels can develop algae problems when pH levels are too high.
Low pH creates different challenges, including equipment corrosion and swimmer’s eye irritation. Unbalanced pH impacts the pool’s overall chemistry significantly, creating an environment where problems compound rapidly.
Lack of Proper Filtration & Circulation System
- Clogged or dirty filters can’t effectively remove algae and debris.
- Many pool owners don’t run pumps long enough. Filters need to cycle all pool water at least once daily, typically requiring at least eight hours of operation.
- Poor circulation creates “dead zones“, or areas where water doesn’t move adequately.
- Broken or malfunctioning equipment prevents proper water turnover. Without adequate filtration and circulation, even properly balanced chemistry can’t prevent algae growth.
Environmental Factors
- Heavy Rainfall: Dilutes pool chemicals while adding contaminants like dirt, debris, and algae spores
- Extreme Heat: Accelerates chemical reactions, causing chlorine to dissipate faster while creating warm water temperatures where algae thrive
- Storm Debris: Introduces algae spores and nutrients
- Pollen: Provides organic matter that algae consume
- Trees & Landscaping: Create ongoing challenges as falling leaves and plant material constantly enter the water
Lack of Regular Maintenance
Skipping routine pool maintenance is perhaps the most common cause of green pools. When you don’t test water chemistry regularly, you can’t identify and correct problems before they escalate. Delayed shock treatments and infrequent cleaning allow algae to establish itself on surfaces where it’s harder to eliminate.
Diagnosing Your Green Pool Problem
Testing Water Chemistry
Before treating your green pool, test your water chemistry using test strips or liquid test kits. For the most reliable analysis, take a water sample to your local pool supply store for professional testing.
These tests should measure free chlorine levels, pH levels, alkalinity levels, calcium hardness, and phosphate levels if available. These results guide your treatment plan by revealing which chemical imbalances need correction.
Inspecting Pool Equipment
- Check your pool filter’s pressure gauge. High pressure indicates a dirty filter needing cleaning or backwashing.
- Examine your pool pump for any unusual noises.
- Look for leaks around pump seals, filter connections, and plumbing.
- Verify proper water flow from return jets. A weak flow suggests circulation issues.
Assessing Algae Severity
Light green water with visible bottom suggests a minor algae bloom that’s relatively easy to treat. Dark green water where you can’t see the bottom indicates severe algae requiring aggressive treatment. Check for algae on walls and floors by brushing surfaces—if clouds of green particles release, algae has colonized these areas.
How to Fix a Green Pool
Step 1: Remove Debris and Brush
- Remove visible debris from the water surface using a skimmer net. Clean skimmer baskets and pump strainer baskets for proper water flow during treatment.
- Thoroughly brush all pool surfaces including walls, floor, steps, and corners to loosen attached algae.
- Vacuum settled debris from the pool bottom. If the pool floor is very dirty, vacuum to waste to avoid returning contaminated water through the filter.
Step 2: Balance the Water Chemistry
Before shocking, adjust pH to the 7.0-7.8 range. Pool chemicals must be balanced before shock treatment because proper pH ensures shock works at maximum effectiveness.
Step 3: Shock the Pool
A chlorine shock applies a high dose of chlorine to the pool, oxidizing algae, bacteria, and other contaminants. Proper shock dosage can be calculated based on your pool’s volume and algae severity. Shock in the evening or nighttime when sunlight won’t degrade chlorine before it can work.
Keep your pump running continuously during shock treatment to circulate chemicals throughout the pool. This ensures shock reaches all areas of the pool.
Step 4: Add Algaecide
After shocking, add algaecide as support treatment. Choose an algaecide appropriate for your pool type and algae variety, following manufacturer instructions. Algaecide works best as prevention and support rather than primary treatment. Shocking does the heavy lifting while algaecide helps prevent regrowth.
Step 5: Run Filter Continuously
Run your pump 24 hours a day until water clears completely. Backwash sand, DE filters, clean cartridge filters frequently during treatment as they trap dead algae and debris. Monitor filter pressure and clean whenever pressure rises 8-10 psi above the clean baseline.
Step 6: Retest and Adjust
Test water chemistry daily during treatment, adjusting chemicals as needed to maintain proper levels. Continue brushing pool surfaces daily to prevent algae from reattaching.
Preventing Future Green Pool Problems
Regular Maintenance Schedule
- Test pool water 2-3 times weekly during normal conditions and daily during heavy use
- Maintain proper chlorine levels consistently and don’t let them drop below 1 ppm.
- Shock your pool weekly during swim season
- Brush and vacuum the pool wall and floor regularly to prevent algae growth
Equipment Maintenance
- Clean filters on schedule based on pressure readings and manufacturer recommendations
- Run your pump at least eight hours a day depending on pool size, bather load, and weather conditions
- Maintain proper water circulation by ensuring all jets flow freely
- Address any equipment issues quickly
Environmental Management
- Use a pool cover when not in use to keep out debris and reduce chemical consumption
- Trim nearby vegetation to minimize falling leaves and branches
- Remove debris when it enters the pool
- Encourage swimmers to rinse off before entering
Professional Help When Needed
- Consider professional pool opening and closing services to start and end each season properly
- Consult with pool professionals when you experience persistent water chemistry problems
- Schedule annual equipment inspections to catch problems before they cause breakdowns during prime swimming season
Preventing Green Pools
Green pool water stems from three main causes: low chlorine levels, poor circulation, and overall chemical imbalance. While discovering a green pool is frustrating, these problems are fixable with proper treatment.
The key to avoiding green pool nightmares is regular maintenance. Testing water consistently, maintaining proper chemical levels, running equipment adequately, and keeping the pool clean can prevent most algae problems before they start. When you notice your pool starting to turn green, act quickly, since early intervention is much easier than treating a fully developed algae bloom.
Keep Your Pool Clear with Net Positive Pools
Net Positive Pool Services provides comprehensive pool services, including weekly maintenance and specialized green-to-clean treatments, to restore your pool’s crystal-clear water. Our experienced technicians diagnose the root cause of your green pool, implement effective treatment protocols, and ensure your water returns to safe, sparkling condition. If you live in North Carolina, South Carolina and the Atlanta area, contact us today to find a Net Positive location near you and get back to enjoying your pool.
