Understanding the Pond Nitrogen Cycle: Why It’s Important to Your Fish

One of the most confusing and intimidating aspects of caring for pond fish care is the science of water quality and the pond nitrogen cycle. You may have heard horror stories about the constant monitoring of pond water just to make sure that the fish stay healthy. Unfortunately, these stories unnecessarily frighten people away from the joy of keeping fish.

While pond water quality is important to the health of your fish, it doesn’t have to be complicated or scary. You don’t need to become obsessive with testing water quality, constantly adding chemicals and doing things to make sure the water is pristine and perfect. There truly can be too much messing around with water. All this to say, keep it simple and your fish will thank you.

Let’s look at the basic cycle of pond water and how it can affect your fish.


The Importance of the Pond Nitrogen Cycle

The nitrogen cycle is a natural process in which different types of beneficial bacteria break down compounds like ammonia and nitrite, reducing them to less harmful compounds like nitrate. This process occurs across three steps, from ammonia, to nitrite, and the final conversion into nitrate.

The first process involves the accumulation of ammonia due to fish waste and decomposing material. As ammonia increases, Nitrosomonas bacteria start to consume it and break it down into nitrite. Once nitrite starts to accumulate, denitrifying bacteria breaks them down into nitrates. Nitrates are then removed from the system by plants, algae, or water changes.

Because so many biological processes are required, it generally takes six to eight weeks for a pond system to run through the nitrogen cycle.

The nitrogen cycle of the pond ecosystem


Ammonia in the Pond

Ammonia is the main nitrogen waste from your fish and functions as the start of nature’s nitrogen cycle. Ammonia is excreted partly by the fish’s kidneys, but mostly by the gills. This is relevant because it calls to mind the fact that if there is damage to a fish’s gills, the fish suffer more than just difficulty breathing. They could have trouble expelling their ammonia too.

High ammonia levels indicate that there are not enough beneficial nitrogen-reducing bacteria living in the pond yet. These bacteria proliferate over time, so ammonia problems are most common during the first six to eight weeks of a pond’s break-in period. Later, ammonia levels can climb again if the filter gets clogged up or a fish dies and starts decaying in the pond. You might see your fish “jumping” out of the pond when ammonia levels get high.

Control of ammonia involves reduced or suspended fish feedings, feeding low-protein food (33-percent or less), and/or upgrading your pond’s filtration. While water changes are the single best remedy for ammonia accumulations that threaten your fish, you can also use Ammonia Neutralizer.


How Nitrite Affects Pond Fish

Nitrite is produced from ammonia in the water by beneficial nitrogen-reducing bacteria. These bacteria cleave off ammonia’s hydrogen ions and replace them with oxygen. The pH of the water is then decreased by the release of these hydrogen ions.

Your fish absorbs nitrites through the skin and gills. Inside the fish, nitrites bind the red blood cells, changing them from red to brown. Brown blood disease results from nitrite accumulations in the system. Nitrite is poisonous at levels as low as 0.1 ppm (parts per million), and fish that die from nitrite poisoning will commonly have flared gill covers.


Nitrate: The Final Product

Nitrate is the final product in the nitrogen cycle. Nitrites are broken down by another beneficial nitrogen-reducing bacteria and become nitrates by this bacterial conversion.

Plants and algae use nitrates, along with phosphates and iron, for cellular growth. When the nitrogen cycle is finished, you will see a sudden bloom of algae on the pond bottom, or a green water explosion that obscures your view of the fish.

This completion of the nitrogen cycle is typically a non-issue for fish health because fish aren’t harmed with acute or sudden exposure. However, with chronic, high levels of nitrate, you may notice weaker fish, slower growth, and increased illness overall. This is because high nitrate levels represent inferior water quality and this causes stress, making fish more vulnerable to disease.

Nitrates should always be less than 80 ppm in an ornamental pond. To reduce nitrates, plants can be added, water changes can be performed, or algae growth can be encouraged. There are tremendous health benefits to a carpet of emerald-green algae on the pond bottom, the greatest of which is nutrition for the fish. This is why we always say that a little bit of algae is a good thing.


Koi in Pond - the Pond Nitrogen Cycle


Testing the pH of Your Pond

Testing water pH is simply a measurement of the free hydrogen ions (H+) in the pond. It is measured on a scale of one to fourteen with anything below seven being acidic and anything above seven being alkaline. The pH required for aquatic life ranges between 5.5 and 8.0. Koi and goldfish can tolerate a very high pH measurement.

Some pond owners spend a lot of time trying to bring down the pH level, but this is unnecessary unless there is also ammonia accumulation in the pond. The toxicity of ammonia is influenced by pH, so at higher pH values, ammonia is more toxic. Below a pH of 7.2, most ammonia is ionized to ammonium and is far less toxic.

The pH level in ponds impacts fish in several ways. First, if it is too low, a condition inside the fish called acidosis results. Symptoms are a loss of appetite and then production of excess slime, as well as isolation and resting on the bottom of the pond. This is followed by a streaking of the fins, and then death.

If the pH is too high (over 10.0), the fish will produce excess slime and gasp at the surface of the water. This condition, called alkalosis, is hard to rapidly reverse once it occurs.


A Word About Chlorine and Chloramine

Chlorine and chloramines are generally added to tap water by municipal water suppliers to make it safe to drink. However, these same compounds are toxic to fish and bacteria and can kill them if exposed for too long. Basically, the chlorine and chloramines burn and destroy the gills of the fish. De-chlorinators such as Pond Detoxifier remove and neutralize these chemicals and should be used whenever more than 10 to 15 percent of the pond’s total water volume is being added.


The Final Verdict

The only way to know for sure what your pond is up to is to test the water for harmful compounds or changing parameters. There is no need to perform regular testing however, especially if your fish level is low. We recommend stocking your pond with no more than one inch of fish per 10 gallons of water. You might want to test the water if you see your fish behaving or looking differently. A quick test can help you identify your next steps for encouraging a healthy environment for your finned friends.

Granted, it may seem intimidating at first, but once you understand how to measure and control pond water factors, it becomes easy and starts to feel like second nature. The bottom line – healthy water equals healthy fish.