Pond Water Chemistry Simplified: pH, Ammonia, Nitrites and More

Pond Water Chemistry Simplified: pH, Ammonia, Nitrites and More

As a passionate backyard pond enthusiast, you naturally aim to create the best habitat for your fish and other pond life. A crucial part of this is understanding water chemistry. 

Getting to grips with the key aspects of water chemistry, including pH, ammonia, nitrite, nitrate, and phosphate, is vital for healthy and thriving koi, goldfish, or water garden pond. 

This article will focus on these essential parameters and highlight the significant role of alkalinity (carbonates) in maintaining the optimal pH levels in your pond. Let’s begin with exploring the pH

Understanding pH Levels in Pond Water: A Key to Fish Health

The pH level in your pond water is a crucial element that significantly influences both the health of your fish and the efficacy of the biological filter. For a deeper insight into pond biological filtration, check out our previous blog post

pH measures water acidity on a scale from 0 to 14, where 7 is neutral. 

Both goldfish and koi are adaptable and thrive in a wide pH range - around 6.8 to 8.0. However, it is important not to overly focus on maintaining an exact pH level. This is because pH levels naturally fluctuate in nature and in your backyard pond. 

The main concern is to avoid excessively acidic conditions, particularly a pH below 6.5. If the pH level in the pond becomes too acidic, it impairs the growth of bacteria vital for the biological filter. This in turn can potentially lead to a build-up of ammonia and nitrite. Such an imbalance can stress your fish, leading to health issues, or in severe cases, death. 

The Role of Alkalinity in Pond pH Stability

Alkalinity, also known as carbonate hardness (KH), is crucial for maintaining stable pH levels in your pond. It acts as a pH buffer, helping to avoid sudden pH changes. If your pond lacks sufficient alkalinity, the water’s pH may become too acidic. As mentioned earlier, this can cause stress and health issues for your fish and disrupt the biological filter.

Regular testing, and if needed, adjusting the alkalinity can help keep the pH at a suitable level. You can measure alkalinity using a test kit. The KH level should be a minimum of 80 parts per million (ppm) to prevent the water from becoming acidic. 


Influence of Tap Water on Pond pH and Alkalinity Balance

Influence of Tap Water on Pond pH and Alkalinity Balance

The tap water you use to fill and top off your pond will have a certain level of alkalinity, determined by its carbonate hardness (KH). The KH can vary based on the source of the tap water, be it from a well, lake, or river, and  its treatment process. This means your tap water might have either a low KH or a reasonable level of alkalinity. 

If the KH in your pond water falls below 80 ppm (parts per million), there is a risk of the water becoming acidic over time. This acidification can occur due to natural processes such as the breakdown of dead algae, leaves, and the activity of the biological filter. 

If you notice low alkalinity in your water, it is important to watch out for signs of low pH conditions. Often, you can manage and stabilize your pond’s alkalinity by simply adding more tap water. However, if the alkalinity consistently drops, consider using an alkalinity booster to correct the situation.  


ph test strips for measuring ph levels in your pond

Daily pH Fluctuations: What to Know?

Pond pH levels are inherently unstable, showing a daily (24-hour) cycle. Typically, the pH is “low” during the night and early morning. This happens because algae and aquatic plants are not removing carbon dioxide (CO2) from the water at these times. 

CO2 in water forms a mild acid, which lowers the pH level. However, as the sun rises and daylight arrives, plants start to consume CO2 via photosynthesis, leading to a decrease in water acidity (and a subsequent rise in pH). Sometimes, the pH may even temporarily rise to 9 in the afternoon. As night approaches, this process reverses, causing the pH to fall again.

Key points to remember are:

  • Pond pH naturally fluctuates within a 24-hour period.
  • pH test results are influenced by the time of day.
  • The optimal time for pH testing is between 9:00-11:00 AM.
  • pH swings, within normal levels, don’t harm the fish.
  • Always check the KH levels, as they have a direct impact on pH stability. 


The Dangers of Ammonia: Prevention and Management

The Dangers of Ammonia: Prevention and Management

You may have been warned about ammonia in the pond, but does a positive test for ammonia mean you need to drain the pond to save your fish? Here’s what you need to know about ammonia. 

Ammonia is a natural byproduct of fish waste, decaying fish food, and the breakdown of organic matter. Chronic low levels of ammonia can cause stress and eventually illness in fish, while high levels can hinder their natural ability to expel ammonia from their organism. Persistent high levels of ammonia is therefore toxic to goldfish and koi, as it damages their gills, and in severe cases, leads to death.  

That said, the risk posed by ammonia is directly linked to the water’s pH level

In water, ammonia is present in two forms: toxic ammonia and harmless ammonium. Ammonia test kits measure the total ammonia content, which includes both forms. 

As the pH level increases, so does the proportion of toxic ammonia. For instance, if your pond water contains 3 ppm of ammonia and has a pH of 8 or above, the ammonia starts to impact the fish negatively. Conversely, at a pH of 7 or lower, the fish are safe from ammonia poisoning. 

Nevertheless, any positive ammonia test at any pH level is a cause for concern and indicates a problem. Regular ammonia testing is key to alerting potential problems early. 

Common causes of rising ammonia levels:

  • Over-feeding the fish.
  • Too many fish in the pond. 
  • Uneaten fish food floating around the pond.
  • Decaying algae or plants.
  • Low pH levels that disrupt biological filtration.


nitrite test kit

Nitrite in Pond Water: Always Toxic?

Nitrite is a crucial parameter to monitor closely in your pond. It forms when beneficial bacteria (part of the biological filtration system), break down ammonia. This process eventually converts ammonia into nitrite, and then into harmless nitrate. 

However, unlike ammonia, nitrite is always toxic, regardless of the pH level! Nitrite impairs fish’ ability to carry oxygen, causing them severe stress and potentially leading to death. 

Nitrite should always be zero when tested. A level of 0.25 ppm is normal during the initial setup of a pond or in spring following a dormant winter, but this should disappear as the biological filter becomes active. If nitrite levels rise to 0.5 ppm, it is crucial to monitor daily, make partial water changes, and investigate the cause. 

Common causes of elevated nitrite levels:

  • Over-feeding the fish. 
  • Too many fish in the pond.
  • The microbes that form the biological filter are coming out of winter dormancy.

Phosphate in Pond Water: Balancing Nutrients for Algae Control

Phosphorus is an essential element for all plants and animals, including those in your pond. This is why balanced pond plant fertilizers and fish foods contain phosphorus. 

In water, phosphorus exists as phosphate. It can originate from underground deposits or be added by the water treatment facility to prevent pipe corrosion. Additional sources of phosphate in pond water include uneaten fish food and fish waste. 

Phosphate is often called an algae-promoting nutrient. Indeed, excessive phosphate will stimulate algae blooms, it is important to note that algae need several other nutrients to thrive. For instance, a lack of iron can inhibit algae growth, even if there is an excess of phosphate. 

Phosphate levels need to be monitored because it is an indicator of overall water quality related to algae growth. A high phosphate level, exceeding 0.25 ppm, suggests a trend toward a nutrient build-up in the water. Typically, phosphate levels will be undetectable with pond test kits. If you’re facing ongoing algae issues, it is a good idea to check phosphate levels.

 Common reasons for detectable phosphate levels:

  • High phosphate content in top-off water.
  • Nutrient build-up in fish-only ponds with no plants.
  • Too few plants in a water garden.
  • Over-feeding the fish.


frog in a pond

Key Takeaway: Water Chemistry in Backyard Ponds Matters

Understanding the basics of water chemistry is crucial for maintaining a healthy backyard pond. You don’t have to be a chemist to understand these concepts!

When you test your pond water, it’s common to find that all is well, which is a sign that you are caring for the pond correctly. The ideal is to have a healthy pond that requires minimal intervention in terms of chemical corrections. To further master your skills in pond water testing, explore our detailed guide here

A good understanding of water chemistry is invaluable, especially when troubleshooting occasional pond issues that might arise. With this new knowledge in hand, you can relax and enjoy the beauty of your pond, confident in your ability to maintain its health and harmony.  

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