New Pond Syndrome: What It Is and How to Prevent It

New pond syndrome is a water-quality problem that can occur when fish are introduced into a pond before the biological filtration is mature enough to process the waste they produce. Ammonia begins to accumulate and, as the biofilter develops, nitrite can also rise to harmful levels.

Despite the name, new pond syndrome is not a single infectious disease. It is mainly the result of an imbalance between the amount of waste entering the pond and the ability of the biological filter to process it. Fish exposed to poor water quality can become stressed, ill and, in severe cases, die.

What is new pond syndrome?

Fish release ammonia as a waste product, and additional ammonia is produced as uneaten food and other organic material decomposes. In a mature pond, microorganisms living mainly on the biological filter media continually process this waste.

A new pond does not yet have a large enough population of these nitrifying microorganisms. When fish are added, ammonia can therefore accumulate faster than the biofilter can process it.

As the first group of nitrifying microorganisms becomes established, ammonia is converted to nitrite. Another group then converts nitrite to nitrate. This means a new pond may experience an ammonia problem followed by a nitrite problem while the biological filter matures.

Nitrate is the end product of normal aerobic nitrification and is considerably less acutely toxic than ammonia or nitrite. It still needs to be managed as part of long-term pond maintenance, but elevated nitrate is not what normally defines new pond syndrome.

Why does new pond syndrome happen?

The most common cause is simply adding fish faster than the biological filter can develop enough capacity to process their waste.

The risk is especially important with koi because they grow large and can produce a substantial waste load, particularly when they are feeding heavily. However, the same problem can occur in any stocked fish pond.

Several factors can contribute:

• adding too many fish to a new pond at once;
• feeding heavily before the biofilter is mature;
• using an undersized or poorly functioning biological filter;
• insufficient water flow through the biological media;
• low dissolved oxygen in the filter;
• low alkalinity or unsuitable pH for effective nitrification;
• cold water slowing biological activity;
• chlorinated or chloraminated water damaging the filter bacteria;
• cleaning or replacing too much mature biological media at once;
• leaving a biological filter without circulation for a prolonged period.

This is also why a similar problem can occasionally occur in an established pond. A pond may not be new, but if its biological filtration is severely disrupted or the fish and feeding load suddenly increases, ammonia and nitrite can again rise faster than the biofilter can handle them.

What happens during the nitrogen cycle?

The biological filter does not simply make fish waste disappear. In a typical aerobic pond biofilter, nitrogen moves through a sequence:

Both ammonia and nitrite can harm fish. The proportion of ammonia present in its more toxic un-ionised form also changes with pH and temperature, which is one reason an ammonia test result should be interpreted together with the conditions in the pond.

The nitrifying microorganisms also need oxygen and suitable water chemistry. Simply installing a large filter does not mean that it instantly has biological capacity. The media first has to become colonised by an adequate population of microorganisms.

For a more detailed explanation of this process, see our guide to cycling a new fish pond.

Signs of new pond syndrome

Fish affected by ammonia, nitrite or other poor water-quality conditions may show behavioural or physical changes, including:

• gasping or spending unusual amounts of time near the surface;
• rapid or abnormal breathing;
• lethargy;
• loss of appetite;
• clamped fins;
• abnormal swimming behaviour;
• irritated or unusually coloured gills;
• flicking or rubbing against objects;
• increased susceptibility to secondary health problems.

None of these signs proves that new pond syndrome is the cause. Gasping, for example, can also result from low dissolved oxygen or gill disease. The correct response is therefore to test the water rather than diagnose the pond from the behaviour of the fish alone.

How to check for new pond syndrome

At minimum, test a new stocked pond regularly for:

• ammonia;
• nitrite;
• pH;
• water temperature.

Testing nitrate can also help show how the nitrogen cycle is developing, while alkalinity is useful because nitrifying microorganisms consume alkalinity as they process ammonia and nitrite.

Ammonia and nitrite should normally be kept as close to zero as practical in a fish pond. A clear-looking pond is not evidence that these compounds are absent: both are invisible and need to be measured with an appropriate test kit.

See our fish pond water quality guide for more information about monitoring pond water.

What should you do if a new pond has ammonia or nitrite?

The exact response depends on the test results and the condition of the fish, but the immediate objective is to reduce the waste load and protect the fish while allowing the biological filter to develop.

1. Do not add any more fish. Increasing the fish load makes the problem harder for an immature biofilter to control.
2. Reduce or temporarily stop feeding. Less food means less ammonia entering the system. Healthy adult pond fish can normally tolerate a short reduction in feeding far better than continued exposure to poor water quality.
3. Test ammonia and nitrite frequently. Do not wait for the fish to show visible signs of distress before checking the water again.
4. Keep the biological filter operating. Maintain adequate water flow and oxygenation through the biofilter. The developing nitrifying population needs oxygenated water.
5. Increase aeration if necessary. Good dissolved oxygen supports both the fish and the biological filtration process.
6. Use partial water changes when required. Replacing part of the pond water can dilute accumulated ammonia or nitrite. Any replacement tap water must first be made safe for fish.
7. Protect the developing biofilter. Do not sterilise the biological media or replace all of it simply because the pond is experiencing an ammonia or nitrite spike.
8. Continue monitoring until the pond is stable.

   To those new to the pond world, these three little words can represent a real problem. New Pond Syndrome (NPS) can occur in the first few months of a pond due to a heavy fish stock and raised ammonia and nitrate levels thanks to poor filtration and increased waste. This can kill fish and really dishearten the fish keeper, causing stress and financial loss.

   New Pond Syndrome can be caused by adding too many fish, too soon to the pond. This creates a build-up of ammonia which can stress and ultimately poison fish. When fish are stressed they are more susceptible to infections, both bacterial and parasitic. So keep an eye out for fish scratching, spots, and other signs of illness which can indicate poor water quality.

   Signs of ammonia poisoning:

   • Fish gasping at the surface
   • Red or purple tinge to the gills
   • Lack of appetite
   • Lethargy, maybe even lying at the bottom of the tank
   • This can be lethal.

   There are several things you can do to avoid NPS. Much the same as an aquarium, the solution is to prepare the water and maintain a very low ammonia, nitrites and nitrates level. Keeping these levels low will also reduce the risk of algae growth.

   Swell UK have a variety of products that will help fight the problem and maintain a healthy pond:

   • When you fill the pond add some de-chlorinator. This will remove chlorine, chloramine and other pollutants from tap water. Any time you perform a water change a simple dose of de-chlorinator will neutralise the water and make it safe.
   • Use a good quality filter than can cope with the amount of water in your pond. The rule usually used is that double the volume of your pond to makes sure you have adequate filtration for a fish pond. So for example, if you have a 3,000 litre pond, opt for a 6,000 litre filter. That way it can cope with waste and debris, and hopefully cut down on maintenance. However, if you have a Koi pond the original volume must be multiplied by 4, as they produce a lot of waste.
   • Use a filter starter in the pond filter. This will give the filtration system a boost of nitrite and ammonia busting bacteria and mature the filtration system. This will start the process more quickly than the natural way.
   • Using a carbon and/or zeolite filter media will also help purify the water and remove any contaminants. Carbon is also well used to remove traces of taints, odours and treatment residues so you can cover many aspects of pond care.
   • Add a few fish at first. Then using a reliable test kit, test for ammonia and nitrites every few days to monitor the correct levels. You will notice a spike in levels at first, this is normal. Then after around three weeks, if the ammonia/nitrite levels have reduced to zero, you can add a few more fish.

   Good pond maintenance is also vital to support fish and plant health. Remove all uneaten food after five minutes, as this will decompose and turn into waste. Also, remove dead plants and sludge from the bottom of the tank, as this is rotting waste and debris that will release toxins into the water.

   With a little time, hard work and perseverance, you can hopefully avoid New Pond Syndrome and enjoy a happy, healthy pond.

   Do not increase feeding or stocking simply because one test happens to give a good result.

Always treat tap water before adding it to the pond

Tap water may contain chlorine or chloramine used by the water supplier for disinfection. Both can harm fish and the microorganisms that the biological filter depends on.

Use a water conditioner or treatment method that is suitable for the disinfectant used by your water supply whenever necessary. Do not assume that leaving tap water standing for a few days will always make it safe: chloramine is deliberately more persistent than free chlorine.

Large water changes can therefore make an existing water-quality problem worse if untreated replacement water damages the fish or the biological filter.

How long does new pond syndrome last?

There is no fixed three-week or four-week timetable.

A new biofilter commonly needs several weeks to develop enough nitrifying activity to handle the waste produced by the fish. Depending on temperature, pH, alkalinity, dissolved oxygen, filter design and waste load, the process can take longer.

The pond should therefore be judged by water-test results rather than by the number of days since the filter was switched on.

During cycling you will often see ammonia rise first. As ammonia-oxidising microorganisms become established, ammonia begins to fall while nitrite may increase. Eventually the biological system develops enough capacity for both ammonia and nitrite to remain very low under the normal fish and feeding load.

Do bottled filter bacteria prevent new pond syndrome?

Commercial preparations containing nitrifying microorganisms can be used to seed a new biofilter and may help establish biological filtration more quickly. They should not, however, be treated as an instant substitute for cycling.

Adding a bacterial starter does not prove that the pond can immediately support a full fish load. Temperature, oxygen, pH, alkalinity and the condition of the product can all influence the establishment of the biofilter.

Whether a starter product is used or not, ammonia and nitrite testing remains the reliable way to determine how the biological filter is performing.

How to prevent new pond syndrome

The safest approach is to establish biological filtration before asking it to support a large fish population.

Fishless cycling can mature a biofilter by supplying a controlled ammonia source without deliberately exposing fish to ammonia and nitrite. Once the filter can process the supplied ammonia and both ammonia and nitrite return to very low levels, fish can be introduced.

If fish are used while a pond matures, begin with a light stock and feed conservatively. Add more fish only after testing shows that the system is consistently processing the existing waste load.

Whatever method is used, continue testing after additional fish are introduced. Every increase in fish biomass and feeding places a new demand on the biological filter.

Choose filtration for the fish load, not just pond volume

The old version of this guide recommended simply choosing a filter rated for twice the pond volume, or four times the pond volume for koi. That is too crude to be a useful universal rule.

A 3,000-litre lightly stocked ornamental pond and a 3,000-litre koi pond with large, heavily fed fish can create very different amounts of waste.

When choosing filtration, consider:

• the actual pond volume;
• the species, number and eventual size of the fish;
• feeding rate;
• mechanical waste removal;
• biological media capacity;
• real water flow through the filter;
• the manufacturer's rating for a stocked pond or koi pond.

For more detail, see our guide to pond filters, setup and filter media.

Should activated carbon or zeolite be used?

Activated carbon and zeolite can be useful pond-treatment media, but neither should replace a properly sized and mature biological filter.

Activated carbon is mainly useful for adsorbing certain dissolved organic compounds, medication residues and other substances. It is not the normal solution for biological ammonia and nitrite control.

Zeolite can adsorb ammonium in freshwater and is sometimes useful as a temporary management tool, but it has a finite capacity and its behaviour is affected by the chemistry of the water. It should not be used as an excuse to continue overstocking an immature pond.

The long-term solution to new pond syndrome is adequate biological filtration matched to the fish load.

Common new pond syndrome mistakes

• Adding a full fish population immediately. A new biofilter does not have mature biological capacity simply because water is flowing through it.
• Following a fixed number of days. A pond is ready when water testing shows that the biological filter is processing the waste load, not because three or four weeks have passed.
• Continuing to feed heavily during an ammonia spike. More food ultimately means more nitrogenous waste for an already overloaded system.
• Trusting clear water. Water can look excellent while containing harmful ammonia or nitrite.
• Adding untreated tap water. Chlorine and chloramine can harm both fish and the biofilter.
• Cleaning the biological filter too aggressively. The microbial population growing on the media is exactly what the new pond is trying to establish.
• Replacing all filter media at once. Removing established biological media can remove a substantial part of the pond's nitrifying population.
• Assuming bottled bacteria make the filter instantly mature. Starter products may help, but water testing must confirm that the filter can actually process ammonia and nitrite.

New pond syndrome FAQs

What is new pond syndrome?

New pond syndrome is a water-quality problem that occurs when fish produce ammonia faster than an immature biological filter can process it. Ammonia and then nitrite can accumulate while the biofilter develops.

Can new pond syndrome kill koi?

Yes. High ammonia or nitrite can injure fish and severe exposure can be fatal. Even sublethal exposure can stress fish and make them more susceptible to other health problems.

How long does new pond syndrome last?

There is no fixed duration. Biological filters commonly require several weeks to mature, and the process depends on temperature, pH, alkalinity, oxygen, filter design and waste load. Use ammonia and nitrite tests rather than a calendar to judge progress.

Can an established pond get new pond syndrome?

A similar ammonia and nitrite problem can occur in an older pond if biological filtration is disrupted, too much mature media is removed, the filter is damaged, or the fish and feeding load is increased faster than the biofilter can adapt.

Should I stop feeding fish if ammonia is high?

Reducing or temporarily stopping feeding is a common immediate response because it reduces the amount of additional waste entering the system. Continue monitoring the fish and water and correct the underlying filtration problem.

Can I add fish as soon as a new pond is filled?

Filling a pond does not mature its biological filter. Source water must first be made safe for fish, and the filtration system should be cycled or the pond stocked very gradually while ammonia and nitrite are monitored.

Does a new pond need a biological filter?

A stocked ornamental pond normally benefits from adequate biological filtration because fish continually produce waste. A fish-free wildlife pond is a different type of ecosystem and does not necessarily require powered filtration.

Is nitrate responsible for new pond syndrome?

Ammonia and nitrite are the main acute concerns during the maturation of a new biological filter. Nitrate is produced later in the nitrification process and still needs long-term management, but it is not normally the compound responsible for classic new pond syndrome.