Ammonia in a koi pond

Ammonia is a wonderful thing when you’re washing your windows. It’s not so wonderful when you’ve got a pond, especially one with fish. Ammonia can be very harmful, even deadly, to koi and goldfish, smells bad and can promote soupy green algae. Luckily, it’s easy to detect and easy to get rid of.

What is Ammonia?

Ammonia is a chemical compound made up of nitrogen and hydrogen. When there are only three hydrogen molecules, it’s called ammonia (NH3). Add one more hydrogen molecule and it turns into ammonium (NH4+). Both forms are interchangeable, taking on their own personality depending on specific characteristics of your pond’s water. For example, in warmer, alkaline water, ammonia will principally be found as the highly toxic free ammonia (NH3). But lower the pH and temperature, and that ammonia will be transformed into the less toxic ammonium (NH4+). Even though NH4+ is less toxic, under certain conditions, all ammonia has the potential to change into the lethal NH3 and so all ammonia should be avoided. Consequently, the only acceptable ammonia reading for a pond is zero.

Fish and ammonia

The most common source of ammonia in pond water is the fish themselves, realeasing most ammonia through their gills. Surprisingly small amounts of ammonia are released in urine and feces. It is impossible to spot ammonia in your pond, so regular testing is essential to keep your pond and fish in good health. If ammonia levels are allowed to build up to lethal levels, your nose will soon detect it. Fish that are experiencing even low levels of ammonia will be found gasping at the water’s surface and will soon lose their appetite.

Fish food and ammonia

One way to keep ammonia to a minimum is to feed koi and goldfish a food that won’t cause them to produce excessive amounts of ammonia in the first place. Goldfish, koi, shubunkins, comets and orfe are all members of the carp family. They have similar nutritional requirements and use their food in similar ways. Carp, like all fish, must eat to meet their energy requirement so

they can at least maintain themselves. Any food they consume over and above their energy demand is surplus and will be used for growth, deposited as reserves, or excreted. In a balanced diet, pond fish consume a complimentary blend of proteins, carbohydrates, oils, vitamins and minerals to satisfy their nutritional requirements. In the wild where protein is usually freely available, carp will feed on a relatively protein-rich diet. Protein is of great value for pond fish,

as it is the only component that they can use for growth. Yet in a protein-rich diet, carp can also afford the luxury of burning the protein as a source of energy, producing the unfortunate by-product of ammonia.

Proteins consist of carbon, hydrogen, oxygen and nitrogen. When protein is ‘burned’ for energy, the nitrogen (N) is released, unwanted, and is excreted in the form of ammonia (NH3). The trick is to offer fish other sources of energy in their diet that will not lead to ammonia. These other sources are carbohydrates and oils. Carbohydrates consist of carbon, hydrogen and oxygen. Carbohydrates are burned a lot cleaner when used as an energy source, and no nitrogen (and

therefore ammonia) is produced as a waste product. One of the reasons why artificial pond diets are so high in carbohydrates is that they provide fish with energy without running the risk of ammonia being produced as a by-product. This is good, because it means that we will experience fewer water quality problems.

Temperature and seasonal implications

Temperature also plays a significant role in the way fish use food and how much ammonia they ultimately produce. In the spring and fall in temperate areas, pond fish should be offered a low protein (20%) food. At these temperatures fish growth and metabolism is reduced. They will simply excrete

a large proportion of the protein being unable to digest it, leading to a build up of ammonia in the pond water. Yet at higher summer temperatures, fish are able to capitalize on a higher protein diet (30-40%), using it more efficiently for growth. So choosing the food you feed to your fish that is appropriate to the pond’s temperature will affect the pond’s water quality through the amounts of ammonia that your fish are likely to excrete.

Managing Food and Feeding

Ammonia excretion is related to the quality and quantity of food offered to our fish. If we offer too much food, or food that has an inappropriately high protein level for our fishes’ particular needs, we are likely to find out later by discovering an ammonia problem. Only feed your fish what they will eat in 5 minutes 2 or 3 times a day, feeding a protein content that is appropriate to the water temperature.

The Nitrogen Cycle

By understanding the nitrogen cycle, we can soon master the art of removing and controlling ammonia levels in a pond. Ammonia reaches the water garden in many ways, whether through fish excretion, decomposition of old fish food and plant material, or chloramined tap water.

The nitrogen cycle is the route by which ammonia is converted to less harmful, and ultimately harmless, nitrogenous components. It takes place throughout the pond, but the high stocking rates of garden ponds require additional biological filtration. A pond’s biofilter is essentially the pond’s living sewerage system that is colonized gradually by the beneficial bacteria. These bacteria require oxygen in order to grow and multiply. Biological filters are installed so that water

runs through them, thereby oxygenating the beneficial bacteria.

Certain kinds of bacteria that are present in the pond and filter (commonly called Nitrosomas bacteria) convert ammonia into nitrites. Nitrites are also harmful to fish and are converted by Nitobacter bacteria into less toxic nitrates which are readily absorbed by pond plants. Certain kinds of aquatic plants, especially cattails, rushes, and pickerel plant, are particularly

good at using these nitrites as food for growth.

New Pond Syndrome

New pond syndrome (NPS) is a term given to a phenomenon whereby ammonia accumulates in a pond to stressful levels, because it’s rate of production is greater than the rate at which it can be broken down by beneficial ammonia loving bacteria.  If the pond is stocked too heavily too soon, not allowing the filter bacteria to reach sufficient densities to cope with the ammonia produced, ammonia will accumulate to dangerous levels within the pond.  The trick is to run in the filter gradually, by stocking the pond with fish gradually.  If you stock too many fish, too soon, you run the risk of losing them all through ammonia poisoning.  There are no short cuts to running in a new pond and filter system.

Things that can cause an increase in ammonia

Pond is stocked with too many fish, too quickly.

Too much food is added to the pond.

Pump failure, preventing water from recirculating through the biofilter

Adding untreated, chloramined tap water.

Fertilizer run-off from lawn or perennial bed


What if I discover I have an ammonia reading?

If a water test shows that you have ammonia in your pond water, you must act immediately, especially if levels are high. Your reading will also most likely be accompanied by a change in your fish’s behaviour (sulking, gasping, loss of appetite etc)– a sure sign that you need to rectify the problem as soon as possible.

1. Stop feeding the fish.

2. Check the biological and mechanical filters to make sure they are not blocked.

3. Perform a 30% water change to dilute the ammonia with treated tap water

4. Add beneficial bacteria to top up the filter’s biological action.

5. Re-test the pond. If the ammonia level is still dangerously high, carry

out another water change, until ammonia levels are safe and on the way down.

6. Feed sparingly and continue testing. If ammonia levels increase again,

return to step 1.

A water change is a quick fix and exists only as a last resort. It’s best to avoid them through wise water quality testing and management. Test your water regularly and be patient when feeding and stocking your pond.

Ammonia removers and binders

Zeolite can be used as an ammonia remover by placing it in a net bag in your filter. There is a limit to how much ammonia it can absorb and will reach a stage where it needs recharging in a bucket of salt water. If you use zeolite in your own pond, be sure not to treat you pond with salt as this will cause the zeolite to release ammonia back into your pond, stressing your fish.


Ammonia levels in pond water can be detected by using a test kit, with the ideal and only acceptable level being zero. As soon as an ammonia reading is detected, corrective action must be taken immediately, for your fishes’ sake.


Carbohydrates consist of carbon, hydrogen and oxygen. Proteins consist of carbon, hydrogen, oxygen and nitrogen. When protein is ‘burned’ for energy, the nitrogen (N) is released, unwanted, and is excreted in the form of ammonia (NH3). Carbohydrates are burned a lot cleaner when used as an energy source, and no nitrogen (and therefore ammonia) is produced as a waste


Kill blanketweed and string algae.