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What mineral turns water green?

Green water is often caused by excess algae growth fueled by high nutrient levels. While several different minerals and compounds can promote algae blooms, phosphates are one of the most common culprits for green pond water.

What Causes Green Water in Ponds?

There are a few key factors that cause ponds to turn green:

  • Excess nutrients – Nitrogen and phosphorus from lawn fertilizers, pet waste, and other sources act like a buffet for algae, allowing rapid growth.
  • Sunlight – Algae need sunlight to photosynthesize and multiply. Shallow ponds and stagnant water receive more light exposure.
  • Warm temperatures – Algae thrive in summer’s warm conditions. Cooler weather in spring and fall limits algae growth.
  • Lack of beneficial bacteria – Beneficial pond bacteria help control algae, but can become overwhelmed by excess nutrients.

Of these factors, excess phosphate is one of the most troublesome in fueling algal blooms and green water. Even very low concentrations of phosphates can set off substantial algae growth.

The Role of Phosphates

Phosphates are a form of the mineral phosphorus, an essential nutrient for plant and algae growth. While phosphates naturally occur in ponds from sources like fish waste and decaying organic matter, runoff from agricultural fields and home fertilizers drastically increases phosphate levels.

Algae and other aquatic plants need phosphates for functions like energy storage and transfer. When phosphate concentrations are high, the algae population can expand rapidly into a bloom. Green algae in particular thrive on phosphate availability.

Some key facts about phosphates:

  • Orthophosphates are the form of soluble reactive phosphates directly usable by algae.
  • As little as 10 parts per billion (ppb) of orthophosphate can trigger algae blooms.
  • Phosphate levels above 100 ppb sustain extensive algae growth.
  • Common sources of excess phosphates include lawn fertilizer runoff, septic systems, and agricultural fields.
  • Phosphates encourage green algal species like Spirogyra, Chlorella, and Oedogonium.

When phosphate levels remain high, algae can rapidly reproduce into green water. Phosphates are readily absorbed by algae, fueling further growth. This allows green algae to outcompete other pond life.

Other Minerals that Contribute to Green Water

While phosphates have a starring role, other minerals and nutrients can also drive green pond water when in excess:


– Forms like nitrate and ammonia; fertilizer runoff is a major source

– Fuels growth of green algae and cyanobacteria


– Abundant in fertilizers, animal waste, and organic debris

– Supports overall plant growth and development


– Can leach from soils and sediments in ponds

– Trace amounts aid algae growth and green color


– Enters through limestone, concrete, mortar, and runoff

– Helps regulate algae buoyancy and mobility


– Added through dolomitic limestone and magnesium sulfate

– Key component of chlorophyll production

While managing phosphate levels is most crucial, reducing inputs of these other minerals through runoff diversion, chemical treatments, and nutrient absorption can help control algae as part of an integrated approach.

Impacts of Green Water

Takeover by green algae can have many detrimental effects on backyard ponds:

  • Blocks sunlight needed by beneficial pond plants
  • Reduces oxygen levels after algae die-offs, harming fish and wildlife
  • Imbalances pH and chemistry in water
  • Can clog pumps and filtration systems
  • Produces unpleasant odors as algae decay
  • Unsightly green appearance

Toxic blue-green algal blooms associated with high nutrient loads can be especially problematic, releasing toxins dangerous to pets, livestock, and humans.

Correcting Green Pond Water

Green ponds can often be corrected through a combination of methods to reduce nutrients while also directly removing algae:

Limit External Nutrient Sources

– Divert fertilizer, soil, and animal waste runoff

– Use phosphate-free fertilizers around pond

– Maintain septic systems; pump tanks regularly

– Scoop out pet waste from areas around pond

Perform Partial Water Changes

– Replace 10-15% of water to dilute nutrients

– Use dechlorinated water if on municipal supply

– Remove water from mid-depth to siphon algae

Treat with Algaecides

– Apply targeted algaecides containing chelated copper

– Ensure product is compatible with fish and plants

– Spot treat visible algae pools first

Install Beneficial Bacteria

– Add bacterial supplements to compete with algae

– Helps establish biological equilibrium in pond

– Look for broad spectrum bacteria strains

Blackout the Pond

– Cover 50% of pond with dark plastic for 3-7 days

– Blocks sunlight to kill existing algae

– Combine with algaecide or bacteria treatment

Using two or more of these techniques simultaneously provides the best chance of bringing the pond back into balance and preventing future algal blooms.

Preventing Green Water Long-Term

Once green pond water is cleared up, maintaining proper pond balance is key to prevent repeat algae outbreaks. Recommended prevention steps include:

  • Test phosphates and nutrients regularly
  • Perform 20% water changes monthly
  • Remove decaying organic matter
  • Limit fish stocking densities
  • Maintain proper filtration
  • Control pond depth and circulation
  • Shade portions of pond from sunlight
  • Plant native grasses and aquatic plants
  • Use phosphate removal chemicals

Vigilantly monitoring pond nutrients, having a customized maintenance routine, and taking quick action at the first sign of green water are the best ways to achieve clear water long-term.

Common Questions about Green Pond Water

What causes green water in my pond?

Excess algae growth caused by high nutrient levels, especially phosphates, is the most common cause of green pond water. Fertilizer runoff, soils, pet waste, and septic systems can contribute phosphates and nitrogen that fuel algae blooms.

What is the green stuff in my pond?

The green color is likely due to microscopic green algae suspended throughout the water, especially Spirogyra, Oedogonium, and Chlorella species. Filamentous green algae may also be visible in stringy clumps.

Is green pond water bad?

Yes, excessive algae can block sunlight, reduce oxygen, alter water chemistry, clog equipment, and look unattractive. Toxic blue-green algae are also a concern. Clear water supports a healthy pond ecosystem.

How do I get rid of green pond water naturally?

Natural remedies include using barley straw to inhibit algae growth, installing water plants to absorb nutrients, introducing beneficial bacteria to compete with algae, and using products containing bacteria or enzymes to digest organic matter.

How do I know if my pond water is healthy?

Indicators of healthy water include clear color, minimal algae on surfaces, oxygen levels above 3 ppm, pH between 6.5-8.5, low ammonia and nitrite, and thriving fish, plants and wildlife.


Green water caused by excess algae is a common pond plague, but with proper understanding of its causes and solutions, clear water can be restored and maintained. Limiting external nutrient sources, physical and chemical algae removal, establishing biological equilibrium, and preventative pond management are key to defend against green water.

Paying careful attention to phosphate levels, along with nitrogen and other supporting nutrients, provides the best odds of stopping algae in its tracks. A multi-pronged action plan to reduce nutrient loads, directly kill algae, and strengthen biological controls can get ponds back on track.

While green water may temporarily turn your idyllic pond into pea soup, knowledge of the contributing factors, corrective techniques, and preventative measures will have you enjoying crystal clear water again.