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Why is My Pond Green?
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Why is My Pond Green?

One day, you're sitting outside by your pond, enjoying the crystal clear water and seeing your fish swim, and the next, your pond is green. Ponds can become green seemingly overnight and without any obvious cause. Green ponds are most common when reopening your pond in the spring and during the warm summer months. Discover why your pond is green, what it means, and how you can treat and prevent green pond water. 

What makes the water green? 

Algae make your pond green. Microscopic algae are common in ponds. You may notice some of the rocks, decor, and sides of your pond have a green fuzz on them from time to time. This is normal. Fish and other pondlife will often eat the algae, and it becomes part of the pond's food chain and ecosystem. 

In the right conditions, algae can start to colonize and bloom--taking over your pond. There are different types of algae that can make your pond green, red, brown, or even blue. 

If you have a brand new pond (or just did a deep clean) and it suddenly is green, new pond syndrome is likely the culprit. This happens when your pond doesn't have enough bacteria in the water to break down nutrients and waste. 

Common types of green algae:

Filamentous algae 

 Green algae that grows like hair in the pond. This type of green algae is not toxic and, in small amounts, is a food source for many fish. Filamentous algae typically attach themselves to water features like waterfalls or the bottom of the pond and eventually break away and float like a clump on the surface. Filamentous algae can grow in colder water, unlike other types of algae. 


If your pond is green and you can't see strings of algae, your pond likely has planktonic algae. These algae are microscopic plants that float in the water. Planktonic algae bloom when they have access to nutrients and direct sunlight and clump together at the pond's surface. 

The issue with algae

A green pond is not aesthetically pleasing and sometimes creates an unpleasant odor. Besides appearances, algae can cause other water quality issues. Algae rapidly absorb oxygen from the water, which can lead to fish and plant kills. When algae bloom, they can block sunlight from reaching underwater plants. 

What causes algae in ponds?

Excess nutrients 

Algae thrive in nutrient-rich environments. When your pond has more nutrients than the filtration system can handle, this can lead to algae blooms. Nutrients can come from fish waste, uneaten fish food, runoff, and organic debris breaking down in the water. 

When your pond has too much nitrogen and phosphate in the water, this can lead to algae blooms. Regularly testing your pond water can help you catch any imbalances in the water before they cause an algae bloom or other water quality issues. 

Warm weather

Algae blooms are more common in warm water. Some algae, like filamentous, can survive in cold water, but many prefer warm water and direct sunlight. The long days of summer can create the perfect conditions for algae bloom.

Warm water also holds less oxygen than cold water. The good bacteria that break down organic material in the water need oxygen to colonize (aerobic). The bad bacteria don't need oxygen (anaerobic) and break down nutrients into nutrients that fuel algae blooms and hydrogen sulfide, making your pond smell like rotten eggs. 

Lack of circulation

Algae need direct sunlight to bloom. When the surface of the water is still, algae get more sunlight. Adding a waterfall, fountain, or other water feature can help keep the water agitated so the algae can't get as much sunlight. This movement also helps prevent mosquitoes from laying eggs. 

How to prevent future algae blooms 


Backyard ponds need filtration systems to remove debris and nutrients from the water. Mechanical filtration uses filter pads to catch debris as it passes through. You can then routinely clean and replace your filter pads. smartpond biological filters use bio-balls to break down nutrients using beneficial bacteria. 

You can combat and prevent algae directly by using UV filtration. UV lights break down algae at the cellular level. 

Reduce nutrients

Algae need nutrients to bloom. Preventing nutrient overload is always a good idea to help keep your water clear. You can scoop out debris from the surface of the pond before it sinks to minimize the work your filtration system has to do. Make sure to avoid overfeeding your fish and overcrowding your pond

Preventative Water Treatments 

Water Clarifier- Use when your pond starts to look cloudy.

Sludge Remover - Use when opening your pond in the spring and when there's buildup at the bottom of the pond.

Barley Pond Clarifier - Use when you want to reduce nutrient loads in the pond.

Pond Tint- Use in the summer to keep your pond cooler and prevent the amount of sunlight that algae can get. 

Water treatments for algae blooms 

If your pond has turned green from algae, you can use smartpond Algaecide and Green Stop Liquid Algaecide. Both water treatments are safe for fish, plants, and wildlife. Always read the labels carefully and use them as directed. These water treatments are fact-acting and help control existing algae and prevent additional algae blooms. 

If your pond suddenly turns green, it's likely the result of an algae bloom. Algae are microscopic plants in the water that use excess nutrients and sunlight to bloom and take over. Small amounts of algae in a pond are normal. Fish and pondlife may use it as a food source. In the warm summer months, with the long days of sunlight, algae blooms become more common. You can help prevent algae blooms by making sure your filtration system is strong enough and reducing the nutrient load in your pond.



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