Why Has My Pool Turned Green This Winter?

By not using your pool in the winter you may expect it to keep exactly how you left it. However, when you return to your pool as the weather gets warmer, you might find that your pool water has, worryingly, turned an unsightly green colour.

If this is the case — don’t worry. A green pool is a normal occurrence and can be cleaned relatively easily. Learn why your pool turns green and how to prevent it in this month’s article.

Pool turned green over winter

Why has my pool turned green?

Green discoloration in a swimming pool is caused by the growth of algae, a type of green aquatic plant that floats on the surface of water. Algae usually flourishes in warm water but can still take over your pool in winter if given half a chance. 

What causes green algae in my pool?

There are various reasons why algae might spawn in your pool. The most common reason is poor chemical balance in the water. Chlorine is typically used to keep algae at bay, but when the level of chlorine drops even slightly, pool algae can quickly start to grow and spread.

Other causes of green algae include:

  • Poor water sanitation
  • Poor water circulation
  • Poor water filtration

You might be noticing a common theme. Algae is typically caused by a poorly managed pool. This is why it is so important to continue looking after your pool even if it’s not being used. Completely ignoring your pool over winter can lead to a whole host of issues.

How to clean algae from your pool

If you’ve returned to your pool to find it infested with algae, you may be eager to use an algaecide. After all, algaecide is a biocide designed to kill algae and prevent its growth. However, we only recommend using an algaecide in the early stages of algae growth, or as a preventative measure. If your pool has been overrun by algae, you should instead follow these steps:

Manual vacuuming

Vacuum your pool on your filter’s waste setting. Don’t rely on automatic or robotic pool cleaners because these aren’t designed for cleaning algae. Make sure you are keeping track of the water level as you vacuum and refill your pool as needed.

Brush your pool walls and floor

After vacuuming you’ll want to scrub the walls and floor of your pool to help chlorine get deeper into any stubborn algae. We recommend using a stiff pool brush on a pole to reach every area of your pool. Pay special attention to the corners of your pool.

How to prevent algae from resurfacing

Prevent algae in pool

Once you’ve cleaned the algae, your next course of action will be to prevent it from respawning. This can be achieved by following these steps:

Test and balance the water

Algae thrives when the chemicals in your pool are unbalanced. To prevent algae from reoccurring, use a testing kit to test the pH of your pool water. You will be able to identify whether you need to add any more chemicals to keep everything harmonious.

Shock Your Pool

Shocking your pool is the process of adding chlorine to the water. This is usually the best way to prevent algae from growing because it struggles to grow in chlorinated water. Avoid using stabilized chlorine when shocking your pool because this adds too much cyanuric acid, which can actually help algae to grow.

Use A Cover On Your Pool

Using a pool cover is one of the simplest but most effective ways to prevent algae from growing during winter. Algae requires sunlight to bloom, and a cover will ensure the pool water remains dark throughout winter. Plus, a pool cover will also prevent other leaves, bugs and debris from getting into your pool. We recommend using a pool vinyl pool cover for preventing algae growth.


Aqua Blue Pools is a leading pool building and servicing business in South Carolina SC. We don’t just design and build your dream pool, we also make sure it remains safe and functional all year round by providing expert servicing and winter maintenance, including cleaning and preventing the formation of algae. Contact our team at any time and we’ll examine and service your pool as needed. 

For all your pool needs, trust in Aqua Blue Pools. There’s a reason we were voted the best pool and spa service. Contact our team today.



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