Seeing your pool water turn green can be frustrating, especially when all you want is a relaxing swim. It doesn’t just look bad. It usually means something’s off with your maintenance routine. If you’ve ever wondered why your pool water is green, it often comes down to algae growth, low chlorine levels, or poor filtration. The good news? It can be fixed. Even better, it can be prevented.
Green pool water is usually caused by algae, poor circulation, or unbalanced chemicals, especially low chlorine. These issues can build up fast if your pool isn’t cleaned regularly or your filter isn’t working well. Fixing it means cleaning thoroughly, adjusting your chemicals, and keeping up with routine care to stop it from happening again.
Common Causes of Green Pool Water
Wondering what is turning your pool water green? There are several common reasons, and most of them come down to water balance, cleanliness, and proper care. Here’s a breakdown of the most likely causes:
1. Algae Growth
Algae is the most common cause of green pool water. It loves warm, still water and thrives when chlorine levels are low. Spores can enter the water through wind, rain, or even swimwear, but they only grow when the water isn’t balanced properly.
According to the U.S. Centers for Disease Control and Prevention (CDC), 92% of harmful algal bloom (HAB) events in 2022 occurred in freshwater sources, and 21% of those were visibly green in color. While your pool is a treated system, this shows how quickly water can turn green when algae are allowed to multiply. Algae overgrowth in recreational water can also carry health risks if not properly managed, especially if chlorine and filtration are lacking.
Keeping your chlorine and filtration systems running properly is key, not just to clear water, but to avoid microbial contamination altogether.
There are different types of algae:
- Green algae are the most common and spread fast.
- Yellow algae (or mustard algae) are more stubborn and grow in shady spots.
- Black algae are tough, cling to surfaces, and need strong treatment.
2. Low Chlorine Levels
Chlorine keeps your pool water clean by killing bacteria and algae. When the chlorine level drops too low, algae grow fast. This can happen after a heatwave, heavy rainfall, or when the pool is heavily used by swimmers.
Also, if your pool lacks stabilizer (like cyanuric acid), sunlight can burn off chlorine quickly, making it less effective.
3. Unbalanced Pool Water
Even if the chlorine is okay, your pool can still turn green if the rest of the pool chemistry is off. pH levels and alkalinity affect how well chlorine works. If the pH is too high or too low, chlorine cannot do its job.
Other minerals, like calcium, can also build up and make the water look cloudy or greenish. That is why testing and balancing your pool water regularly is so important.
4. Poor Circulation or Filtration
Your pool filter and pump play a big role in keeping the water clean. If the circulation system is not working properly, algae and bacteria can build up. A clogged or dirty filter will not remove dirt or organic material.
If you are not running your pool pump long enough, especially in hot weather, the water becomes more prone to turning green.
5. Debris and Organic Material
Leaves, bugs, dirt, and other debris can build up in your pool and feed algae. If you do not clean your pool regularly, the organic matter begins to break down, affecting water clarity and inviting algae to grow.
A quick daily skim and weekly vacuuming can make a big difference.
How to Fix Green Pool Water
Now that your pool water is already green, it is time to fix it. Don’t worry, with just a few steps, you can get your swimming pool looking clean again.
1. Test the Water
First, check the chemical levels. Use a reliable pool water test kit or test strips to measure chlorine, pH, and alkalinity. This helps you understand what your pool needs.
If the chlorine is at zero and the pH is off, you will need to adjust both before moving forward.
2. Shock the Pool
Shocking means adding a large amount of chlorine to kill off algae and bacteria. Use a chlorine shock treatment and follow the instructions on the label. The greener the water, the more shock you will need.
Do this in the evening when the sun cannot burn off the chlorine, and keep your pump running so the water circulates well.
3. Brush and Vacuum Thoroughly
After shocking, use a pool brush to scrub the walls, steps, and floor. This helps loosen the algae so it can be filtered out or vacuumed up. Then, vacuum your pool to remove dead algae and debris.
A manual vacuum works better in this situation than an automatic one.
4. Clean or Backwash the Filter
Your pool filter will probably be full of algae after this process. If you have a sand or DE filter, backwash it. If you use a cartridge filter, clean it well or replace it if necessary.
A clean filter helps prevent algae from returning.
5. Run the Pump Continuously
Let your pool pump run continuously for 24 to 48 hours to help clean the water thoroughly. The constant circulation helps the filter catch and remove leftover algae particles.
6. Rebalance the Water
Once the green is gone, retest and balance your pool water. Adjust your pH, alkalinity, and chlorine to ideal levels. This makes the water safe for swimming and keeps it from turning green again.
How to Prevent Your Pool from Turning Green
Fixing a green swimming pool takes time, but preventing it is simple with the right habits. Here’s how to keep your pool water clear and blue:
1. Maintain Proper Chlorine Levels
Check chlorine levels regularly, especially after big pool parties or hot weather. Chlorine should stay between 1 and 3 parts per million (ppm). Add a chlorine stabilizer to protect it from sunlight.
2. Test and Balance Water Weekly
Use a test kit once a week to monitor pool chemistry. The ideal pH is between 7.2 and 7.6. Alkalinity should be between 80 and 120 ppm. Keeping these balanced helps your chlorine work properly.
3. Keep the Pump Running
Run your pump at least 8 to 12 hours a day during hot months. Good circulation keeps the water clean and reduces the chance of algae forming.
4. Clean the Pool Regularly
Skim leaves, bugs, and dirt from the water daily. Brush the walls weekly and vacuum as needed. This stops debris from breaking down and feeding algae.
5. Use Algaecide for Extra Protection
Consider using a weekly algaecide treatment, especially during hot or rainy seasons. It gives extra protection, but should never replace regular chlorine.
When to Call a Pool Professional
If your pool water stays green despite trying everything, it may be time to call a pro. A professional pool service can help with serious algae problems, broken equipment, or unbalanced water that’s hard to fix on your own.
Sometimes, expert help saves you time and money in the long run.
Keeping Your Pool in Check
A green pool might look alarming, but it’s usually something you can handle with the right steps. In most cases, it’s the result of low chlorine, poor filtration, or unbalanced water chemistry. By knowing what to look for and staying on top of regular maintenance, you can keep your water clear and swim-ready.
Stay in the habit of testing your water weekly, cleaning your filter, and running your pump consistently. These small steps make a big difference. And if something doesn’t feel right, it’s always okay to get a second opinion or professional advice.
If you’re thinking about building a new pool or upgrading your current one, BC Pools specializes in custom-built pools that are designed for quality, durability, and easier maintenance. A well-built pool from the start can help prevent many of the common issues pool owners face.
Contact us today to learn more or get a free quote. Your dream pool starts with the right team behind it.
With consistent care and a reliable foundation, your pool stays clear, clean, and ready whenever you are.
