Pond Info Center > The Nitrogen Cycle of a Garden Pond

The Nitrogen Cycle of a Garden Pond

Keeping a garden pond, like most other worth-while tasks, is both an art and a science. In this article, we will look at one of the most important scientific aspects of pond life, namely the nitrogen cycle. But don't be fooled! That does not mean that the information here is of no practical use. On the contrary, a basic understanding of the nitrogen cycle is actually vital to good pond keeping, and can mean the difference between life and death for your fish. On the positive side, cycling a pond is quite simple, since nature does most of the work for you, and the basics are quite easy to understand.

Nature's Role in the Nitrogen Cycle

Fish, like people, produce the nitrogen waste ammonia as a by-product of digesting protein. Some ammonia is released in a fish's urine, but for the most part it is excreted from the gills. A potential problem now arises, because the ammonia, which is toxic, is released directly into the same water where the fish live. If the ammonia level in a garden pond or aquarium rises high enough the fish can become stressed, or even die.

Nature, though, when allowed to run its course, safeguards against the ammonia reaching a dangerous level. An ammonia-eating bacteria called nitrosomonas converts the ammonia (NH3) into nitrite (NO2). Nitrite, however, is still toxic. But fear not! There is another bacteria, nitrobacter, that turns the toxic nitrite (NO2) into nitrate (NO3), which, except at extreme levels, is safe for fish. Nitrobacter and nitrosomonas are naturally present everywhere, so you don't have to add them to the pond. The beneficial bacteria don't do to well below 55 degrees Fahrenheit, so it is best to start a new pond in the spring, which is when most people start new ponds anyway.

Your Part in the Nitrogen Cycle

Now that we've discussed what nature does to balance pond life, let's take a look at what is required on the human side. Actually, for the most part what is required is patience; waiting until the right time to add new elements to the pond, and then waiting for nature do its work.

After building your garden pond, the first step before adding any plants or fish is to circulate water for about two weeks. During this period, you should test the water to confirm a safe pH and find out if any chemicals that are harmful fish, such as chloramine, are present in the water. A good pond test kit will tell you what to look for and how to make any necessary adjustments. You should also use this time to look for any leaks or mechanical problems, both of which will be much easier to fix before any plants or fish are added.

Once the water is adjusted, the next step is to introduce pond plants to the garden pond. Plants eat ammonia, nitrite, and nitrate, so the more plants you have the smoother the nitrogen cycle will be.

Ideally, you should wait a month after adding plants to the pond before introducing fish, and once this stage begins the fish should be added one at a time. The rationale behind spacing new fish is that the bacteria need time to reproduce in response to the ammonia excreted by each new fish. If too many new fish are added at once, or if the pond becomes overcrowded with fish, the ammonia level will become hazardous.

When adding new fish, you should measure the pond's ammonia and nitrite levels on a daily basis. As new fish are added you will probably observe a rise in ammonia levels for about a week. This rise is normal, and it is necessary to establish the nitrogen cycle, so you should work to lower the ammonia only if it reaches a level that your test kit indicates is dangerous for the fish. Following the week long rise in ammonia, the bacteria will be in a good position to go to work and the ammonia levels should begin to decline.

Like we said at the beginning, nature is the primary mover in the nitrogen cycle and pond life. Mostly, you just have to have a little patience and watch carefully.

Related pond article: Fixing Pond Cycle Problems

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