How to Cycle a Pond for Fish (Step-by-Step)

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several koi benefit from a fish pond being cycled
New ponds need to be cycled before adding koi or goldfish to ensure adequate bio-filtration can take place.

Pond cycling is a term used to refer to the natural cycle that occurs within a pond to maintains its ecosystem – the nitrogen cycle. This cycle is a very important process that is necessary to understand if you want to keep fish, as this forms the basis of a healthy pond environment. Before adding fish to a pond, it’s important to cycle the water through your pump and filter box to allow this natural process to establish, which is the ultimate goal of pond cycling.

The nitrogen cycle, it simplest terms, is the breakdown of harmful organic substances into less harmful compounds. For example, fish within a pond produce waste, and this in turn raises ammonia levels in the water as a by-product. Ammonia is very dangerous to pond fish if levels get too high, and so this needs to be removed to ensure fish health.

Natural beneficial bacteria living in your pond will provide this function, breaking down these organic substances into other compounds which are readily used by plants. This process is something that will occur naturally in all ponds, and the process will improve and become more efficient as beneficial bacteria levels grow over time.

Problems can occur, however, when you have a brand new pond and want to add fish to the system, as there will be no beneficial bacteria yet to begin processing harmful waste and organic substances. Fish introduced into this kind of pond will likely become sick from the slowly rising ammonia and nitrite levels.

This is where pond “cycling” comes in! Cycling refers to the process of circulating pond water in a fresh pond using your pump and filter until you obtain an effective level of beneficial bacteria for biological filtration (the nitrogen cycle) to take place. Once bacteria levels are adequate, you can safely add fish to your pond and you will have enough beneficial bacteria to break down waste and harmful substances effectively. This will result in better fish health, growth, and an overall more balanced pond environment.

Do I NEED to Cycle My New Pond?

No, but you really, really should! As explained above, a new pond will not have enough biological filtration (bacteria) in place to safely break down fish waste and organic debris. Introducing fish to a fresh pond system will only cause harm to the fish, and will even slow down the natural cycle from becoming established due to substance imbalance that will occur.

Not only that, water that is freshly added to a pond is likely tap water and packed with high amounts of chlorine, a heavy organic pollutant. Chlorine is also dangerous to fish, and on top of the lack of biological filtration and heightened ammonia, they will also suffer high levels of chlorine found in untreated tap water.

Basically, if you intend to have fish, and care about your fish, you owe it to yourself to cycle your pond!

What If I Don’t Have Fish? 

A pond without fish still benefits from being cycled
A pond without fish doesn’t need to be cycled, but will still benefit from the process.

If you don’t intend to have fish, cycling your new pond water is less of a concern. Since fish waste is the primary reason why high levels of beneficial bacteria are needed from the get-go, removing this component means a pond can have more time to become naturally established. Beneficial bacteria that will provide your pond with biological filtration will naturally begin to populate your pond as soon as the water touches the pond liner. Bacteria will arrive from the air, rain, wind, and even surrounding soil. Since no fish will be present, bacteria will have plenty of time to grow, but it can still take weeks or months for the process to become efficient.

Even without fish present, there will still be organic waste that eventually needs processing. Leaves, grass, insects, and small organic particles that enter your pond will soon start to decompose and settle on the bottom of your pond. This organic matter becomes something we call “sludge” or “silt,” and beneficial bacteria is what is needed to break this down. Excess sludge can cause issues with water clarity, increase algae growth, and also give your pond an unpleasant odor.

If you have issues with debris entering your new pond, with or without fish, it may be good to supplement with a concentrated beneficial bacteria product to kick-start bacteria populations.  This will greatly reduce the time needed for biological filtration to take place, allowing for much faster waste, sludge, and organic matter breakdown.

The Pond Nitrogen Cycle & Beneficial Bacteria

pond plants naturally cycle water and nutrients in a pond
Pond plants will absorb nitrates, using them to grow, and produce oxygen as a result.

Before we talk about the most efficient way to cycle a new pond for fish, let’s discuss first some information about the nitrogen cycle and some important facts.

As mentioned above, the nitrogen cycle is the breakdown of organic substances into their constituent inorganic compounds. Within a mature fish pond with a nitrogen cycle in place, waste produced by fish and organic matter that sinks will begin to decay. The bacteria that break down the organic matter and cause the process of decomposition produce ammonia as a by-product.

Ammonia is toxic to living organisms, and will harm your fish if levels are allowed to rise too high. Luckily, there is one type of bacteria, called nitrosomonas, which love to break down ammonia in your pond water. Using dissolved oxygen in your pond, nitrosomonas feed on ammonia and release another chemical called nitrite.

Nitrites are also harmful for fish if left unchecked, so nitrites also need to be removed by another type of microorganisms, called nitrobacter. These bacteria reduce nitrites into much less harmful nitrates, which are readily used by pond plants and algae, or reduced with filtration and water changes. This forms the basis of a pond’s nitrogen cycle, which in a nutshell would be:

Organic waste  (bacteria) -> Ammonia (nitrosomonas) -> Nitrites (nitrobacter) -> Nitrates (plants)

What are ideal substance levels in a pond?

A pond’s nitrogen cycle will be a continually changing process, becoming more and less efficient as certain things happen. If there is an algae bloom in summer, for example, this will likely reduce oxygen in your water that nitrosomonas bacteria need to break down ammonia effectively. This will lead to higher ammonia levels, less oxygen, and increased organic waste.  On the other hand, a large amount of bog plant growth will increase oxygen levels and provide beneficial bacteria a spike in production, reducing organic waste as a result.

There are also things outside the nitrogen cycle that can affect the balance of your pond which also need to be considered, such as chlorine from tap water. This isn’t something bacteria can break down or remove naturally, but it is still something that can seriously harm your fish unless treated. You can test pond water for substance and pH levels using test kits designed for ponds. Below we have listed some of the more important things to consider when cycling and testing water quality:

Ammonia: The ideal value for ammonia in a fish pond is zero to 1 part per million (ppm). The less ammonia present, and the lower you can get the value, the better for your fish. Beneficial bacteria populations will help remove ammonia, and starter populations can be achieved with correct water cycling.

Nitrite: The ideal value for nitrite in a fish pond is zero to a level lower than 5 ppm. Nitrite is almost as harmful to fish as ammonia, so the lower the nitrite levels, the better. Beneficial bacteria  will help remove nitrites, and starter populations can be achieved with correct water cycling.

Chlorine: The ideal value for chlorine in a fish pond is zero. Chlorine is a heavy pollutant and extremely toxic to fish. Water should be dechlorinated during the cycling process and before fish are added to the pond.

pH: Ideal pH for Koi and Goldfish is 7.0 – 8.0. Small fluctuations outside this range is generally harmless, but maintaining this range in the long term will provide your fish the most benefits.

During the cycling process, and after fish have been introduced, there are ways to enhance the process making it more efficient and obtaining more desirable values. Steps to help promote a healthy nitrogen cycle during and after cycling have been listed below.

Steps to Cycle a Pond That’s Ready for Koi & Goldfish 

Step 1: Removing Chlorine From Water (2-5 days)

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If you’ve just started a new pond, or have carried out a heavy water change in your current pond, chances are you have used water from your mains supply. Mains water, or tap water, is disinfected with chlorine to remove harmful bacteria to make the water safe for drinking.

Although most chlorine is removed before it can reach our supply, residual chlorine is still left behind in the majority of water. This is fine in low quantities for us, but can be extremely harmful to pond fish. Chlorine as a chemical has a nasty erosive effect on a fish’s gill tissue, causing issues with breathing over time. It also decreases the color of fish, and will increase stress as fish try to constantly move to less chlorine heavy water.

Water should ALWAYS be treated to remove the bulk of chlorine before adding fish to the pond. Chlorine cannot be removed by beneficial bacteria, and high levels can actually inhibit bacteria growth, so it needs to be removed preferably before bacteria has time to establish.

The process of removing residual chlorine from tap water is called dechlorination, and there are a range of treatments you can use to help. These should be used as the first step of the water cycling process, or after a water change, as the lower chlorine levels will help beneficial bacteria growth in the second step. Below we have listed our favorite dechlorinator products for effective chlorine removal in garden ponds, both of which are safe for fish and plants.

If there is still a bit of residue chlorine left after treatment, you can add activated carbon to your filter system to help reduce the chlorine as close to zero as possible.

Step 2: Raising Beneficial Bacteria (4-6 weeks)

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The second step to successfully cycle a pond for koi or goldfish is the allow time for beneficial bacteria to grow so biological filtration can take place. Biological filtration is absolutely essential for a healthy pond, and beneficial bacteria populations need to be active before you can introduce fish to your water. Naturally, it will probably take around 4 weeks for a new pond and filter box to achieve a reasonable beneficial bacteria population – which is quite a long time!

You can drastically speed up this process by supplementing new pond water with a natural, concentrated beneficial bacteria supplement. Most products will use the same bacteria strains that would be found naturally in a pond, and since you cannot overdose on beneficial bacteria, supplementing has no real drawbacks. Beneficial bacteria supplements will not only help kick-start a new pond’s biological filtration, but it is also a great thing to add after any kind of water change. Since we want ammonia levels as close to zero as possible for the best fish conditions, the more beneficial bacteria available, the better.

Adding a concentrated beneficial bacteria product will also massively reduce the time it takes for bacteria to colonize, with some bacteria become established almost immediately after application. Since beneficial bacteria will still need some organic matter to grow efficiently (they need to eat something!), you can add small amounts of fish feed or debris during the cycling period to produce small amounts of ammonia to kick-start the population.

It usually takes around 6 weeks to cycle a pond normally, but this can be reduced to 4 weeks with the use of beneficial bacteria products and the addition of small amounts of waste (bacteria food). We have also listed below our top picks for kick-starting a pond when cycling, and you can read more about beneficial bacteria here.

Step 3: Testing Substance Levels

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After removing the bulk of chlorine residue from your pond water, and allowing time for beneficial bacteria to grow within your pond and filter for biological filtration, it’s time to test the water to ensure ammonia, nitrite, and pH levels are within a good range. This is something that should be carried out before and after adding fish, with a monthly test being good practice to ensure a healthy pond environment.

Very low ammonia levels, nitrite levels, and a pH in the range of 7.0-8.0 is ideal here. A good ammonia range is from 0 to 1 part per million (ppm), and a nitrite level of less than 5 parts per million (ppm).

Simple test kits designed for ponds can provide instant and accurate results, and we have listed some below for convenience which display values in ppm.

Step 4: Introducing Pond Fish 

After a few weeks of cycling your pond water, removing chlorine, and allowing beneficial bacteria to grow, it’s probably time to add your fish! If your water tests are coming back with good results, and there are no spikes in harmful substances, fish can be introduced into the water safely.

After fish are introduced, we recommend performing monthly tests to monitor water pH, ammonia, and nitrite levels and to supplement with beneficial bacteria or activated carbon when needed.

How to Maximize Pond Conditions After Cycling

1) Regular Filter Maintenance 

To ensure the best water quality and the minimal amount of debris buildup, regular cleaning of a filter’s mechanical media is recommended. Mechanical media works to remove larger debris and particles, whereas biological media is optimized to break down harmful organic substances, such as ammonia.

Biological media doesn’t need to be cleaned, as this would remove the beneficial bacteria populations responsible for bio-filtration. Mechanical media, however, such as foam or pads, will eventually become clogged and less effective over time. Cleaning your mechanical media every month or two is recommended for maximum filtration results.

Even though it’s not recommended to clean biological media, some media is better optimized for bacteria than others. Adding higher quality biological media can achieve better filtration results as there is more surface area for bacteria to colonize. For more information on the best media available, check our article here.

2) Keep Algae in Check

too much algae impacts the nitrogen cycle
Too much algae lowers dissolved oxygen levels and impedes a pond’s ability to naturally cycle itself.

All ponds will have some amount of algae growth, but if left unchecked, a heavy algae bloom can wreak havoc on a pond’s ecosystem. Algae compete with beneficial bacteria for oxygen, and the more algae in your pond, the less oxygen will be available to both bacteria and fish. As dissolved oxygen lowers, beneficial bacteria work less effectively to break down ammonia and levels begin to rise.

If you notice your pond looking green (like pea-soup), then you should consider adding a UV clarifier to your pond circuit to remove the bulk of the free-swimming algae. UV clarifiers are safer than chemical algaecides and just as effective at removing most types of algae. For larger algae, such as string algae, these will need to be manually removed with a pond net or vacuum cleaner.

3) Remove Excess Pond Sludge

No matter how good your beneficial bacteria population, you will probably still have a gradual build up of pond sludge. Pond sludge in low quantities is perfectly fine for most ponds, and is not harmful for fish. Problems occur when sludge levels increase, as the bacteria which break down sludge also consume oxygen and nutrients from the water. This leads to a reduction in water quality and dissolved oxygen, which can harm koi and goldfish. If your pond sludge is slowing build up, consider adding a natural remover or manually removing the bottom much. Check our article here for more information on this.

4) Select a High Quality Fish Food

Not surprisingly, low quality fish feeds will result in more waste as the bulk of these feeds contain “filler” content that fish cannot process. More waste means higher ammonia, and a feed with less nutrients means fish aren’t receiving maximum benefits from their food intake. Poor quality fish feeds can cause fish to become sick, stunt growth, and even cause a decrease in color. Not all fish food is made equal, and for more information on how to select a high quality feed and what feeds we recommend, check our article here.

5) Regularly Check Water Condition

activated carbon helps cycle water
Regularly check your water quality, and add activated carbon as needed to neutralize pollutants like ammonia.

Even with everything above, sometimes things happen which cause an increase in ammonia, nitrites, or a change in water pH which you just cannot control. The best way to handle this is to regularly monitor water quality with test kits, taking note of substance levels each month.

If ammonia is spiking it could be beneficial bacteria levels have taken a plunge, so supplementing more bacteria more may be required. If water pH has changed there could be a pollutant entering your pond, such as an airborne pesticide, so adding activated carbon can help bind and remove this pollutant.

Keeping on top of water condition and working to prevent a problem before it can effect your fish is the best method of safe fish keeping. As soon as you notice an issue, work to resolve it before it can take hold!

Chris G
About the author

Chris G

Pond consultant and long-time hobbyist who enjoys writing in his spare time and sharing knowledge with other passionate pond owners. Experienced with pond installation, fish stocking, water quality testing, algae control and the troubleshooting of day-to-day pond related problems.

Read more about Pond Informer.

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