How to Lower Ammonia In Fish Ponds (And Keep it Low)

How to Reduce Ammonia Levels in Ponds with Koi & Goldfish 2018

lower ammonia koi pond
Ideally pond ammonia should be as close to zero as possible, as the chemical is very toxic to both goldfish and koi.

If you have a pond with goldfish or koi, you will also always have some level of ammonia; a constantly produced by-product of fish waste that just can’t be avoided. Our job as fish keepers is instead to keep the levels of ammonia as low as possible, whilst also maintaining good and stable overall water quality.

Ammonia will actually be found in all types of ponds, even those without fish, as the substance is produced during the decomposition of organic waste. In ponds without any fish, the waste would come from rotting leaves, dead insects, plants, or even visiting ducks and herons. Rarely in a pond without fish would ammonia be able cause major issues, as the ponds natural beneficial bacteria population would be able to break down the low quantities before they could rise to harmful levels.

However, in a fish stocked pond, beneficial bacteria can’t always keep up with the bio-load and waste levels, so extra steps need to be taken to help keep ammonia as low as possible. Ideally, ammonia should be zero, but realistically this isn’t possible as there will also be fresh ammonia introduced by fish on a daily basis. Keeping ammonia as close to zero as you can is a primary goal in fish keeping, as this creates a much healthier environment for fish with less risk of rapid water quality changes or problems.

How do you Test for Ammonia in Pond Water?

The only way to know for sure if you have high ammonia in your pond is to test your water using a testing kit, but there are also some common signs that could point to a sudden spike. For example; cloudy water, algae problems, or sick fish could all point to a rise in ammonia or decline water quality that needs to be investigated.

Changes in water clarity could indicate a decrease in water quality, which can be determined by testing pond water.

Testing pond water for just ammonia is rare, and you’ll find most commercial test kits provide measurements for a range of different values; including ammonia, nitrites, nitrates, pH and KH. Even if tests for just ammonia were common, it would still be more helpful to perform a broad range test as it gives a better indication of overall water quality. Not only this, KH (carbonate hardness) influences pH by acting as a buffer, and pH directly affects the toxicity of ammonia in water. In other words, if KH crashes, ammonia could spike, which would cause even small amounts of ammonia to become extremely toxic. Even if you tested for ammonia and the value was fairly low, how can you be sure your pH is ideal and not contributing to more potent waste substances?

Testing your water for a range of values is always good practice, and luckily this can be carried out easily using commercial water testing kits. We recommend API Pond’s Master Test kit as it provides a great range of tests and is more accurate than strip based testing kits. For more information on pond water testing, check our dedicated article here.


What causes High Ammonia in Fish ponds? 

1) Poor Water Filtration & Insufficient Bacteria

The more fish you have in your pond, the larger your bio-load (waste), and the more filtration you will need.

If your tests come back indicating high ammonia levels, the first thing to check is your filtration system and overall pond bio-load. Pond water should be cycled once every hour through your filter box for optimal filtration, and your filter and pump need to be large enough to handle this turnover. If you’re running a 2,000 GPH pump in a 4,000 gallon koi pond, for example, the pump is not going to be able to cycle the water efficiently. Likewise, if your filter box is too small there may not be enough water floor or capacity for sufficient bio-filtration to take place. The more fish you have in your pond, the better filtration you need to remove harmful substances!

Along the same lines of filtration, ponds without sufficient beneficial bacteria populations will have issues with high ammonia. Beneficial bacteria is essential to the nitrogen cycle, which breaks down harmful ammonia into nitrites, and then nitrites into nitrates which are used by plants and algae.  Without beneficial bacteria ammonia has no where to go, and will gradually rise to dangerous levels. New ponds often have issues with this when they’re not cycled correctly, as well as mature ponds where filter boxes are not cleaned regularly and bacteria die.

2) Excess Waste, Sludge, and Poor Quality Food

Sludge at the bottom of your pond is all the organic waste that collects in your water and sinks and accumulates on the pond liner. This includes fallen leaves, twigs, insects, fish waste, and plant matter. During decomposition ammonia will be produced which needs to be broken down by bacteria, but sometimes waste just builds far too much and they need a helping hand. Regularly cleaning bottom muck in a fish pond is good practice, and will help alleviate some of the bio-load and improve water quality.

Another surprising cause of gradually increasing ammonia which is often overlooked is feeding your koi or goldfish poor quality foods. “Budget” fish feeds will contain a large amount of filler content, such as Ash and bulking agents, which are used to cut down on overall costs. The problem with this is that fish can’t make use of these ingredients and it all passes straight through them back into your pond! Fish fed with low quality foods will produce more ammonia and waste content in comparison to a high quality feed.

3) Lack of Water Flow, Oxygen, and Aeration

Oxygen is essential for ponds and bio-filtration, and a lack of oxygen means beneficial bacteria cannot function efficiently. Bacteria require oxygen and nutrients to break down ammonia, so a pond should be optimally aerated to provide the maximum amount possible. Stagnant ponds with low water flow will have all sorts of issues, with rising ammonia levels being just one of them.  If you have fish in your pond, you owe it to them to provide sufficient aeration to the waters, as they can easily become sick in low oxygenated conditions.


Getting rid of Ammonia in your Koi or Goldfish Pond (Fish Safe methods)

1) Ensure Filtration is Adequate & Optimized

Filters and pumps should match your ponds bio-load for optimal water filtration.

Ammonia build-up is most common in fish ponds that do not have adequate filtration, and where the ponds bio-load (waste levels) exceed filtration capacity. Pond water needs to be cycled through a filter box every hour to maintain water quality, so making sure your pump is strong enough and filter box large enough is the first thing to check. If your pond is 4,000 gallons you’ll need a minimum of a 4,000 GPH rated pump and a filter box that supports this flow rate. The more fish you have, the better filtration you will need to provide, and since you can’t ‘over-filter’ a pond, the bigger the better!

Making sure your filter box and filter media are regularly cleaned is important, too, as well as optimizing your media for the maximum amount of debris removal and bacteria populations. Bio-media should be chosen for maximum surface area for the largest amount of bacteria colonies, and mechanical media should be layered with course, medium, and fine holes for maximum debris capture. Having just a single layer of mechanical media is often not enough for fish stocked ponds if you want to maintain lower ammonia levels.

  • For more information on top quality filter boxes, check here.
  • Learn more about how to clean and optimize filter media in our article here.

2) Supplement Extra Beneficial Bacteria

Supplementing with extra beneficial bacteria can help reduce the amount of ammonia and nitrites in water.

Beneficial bacteria is at the very heart of the nitrogen cycle, and without beneficial bacteria ammonia could not be removed from your water. Most ponds as they mature will establish strong bacteria populations and won’t need to be supplemented, but in some cases it can give bio-filtration a boost. If your water tests are coming back with high ammonia readings it means your bacteria are struggling to breakdown the excess, so adding more will only improve water quality. Even though the issue is unlikely due to low bacteria in a mature pond, supplementing with concentrated bacteria can help reduce ammonia while you try to solve any other problems (poor filtration/aeration etc.) Bacteria products are often added directly to the filter box or into the pond water, and will get to work quickly breaking down ammonia.

They’re very useful in newer ponds especially where beneficial bacteria has not become established, and are a key part of cycling a new pond ready for fish. For more information on the best bacteria and cycling, check our article below:


3) Provide Oxygen and Aeration

Dedicated air pumps are great for providing oxygen for larger ponds or larger fish, such as koi carp.

Bacteria can’t function without oxygen and will struggle to break down ammonia and nitrites in low oxygen conditions. Likewise, both goldfish and koi require plenty of dissolved O2 to stay healthy and will thrive in well aerated waters. A rise in ammonia could be related to stagnant water with low oxygen, so adding aerators will likely help improve conditions. Waterfalls and fountains provide natural aeration and oxygen to the water, and come in both electric or solar powered varieties. For koi ponds or heavy stocked ponds, adding a dedicated air pump can be great for a constant and stable supply of oxygen all year, providing huge doses of oxygen to both fish and bacteria. Air pumps are especially useful during winter if your pond freezes over, as they help maintain holes in the surface ice and keep a constant supply of oxygen going during the coldest months.

Combining waterfalls, fountains, and air pumps together would be ideal, but any aerator is better than none in a fish pond – so take your pick! The more oxygen you can provide, the better.


4) Deter Common Pests & Predators

Ducks may seem cute, but they bring parasites, diseases, and huge amounts of potent ammonia to ponds.

Ducks, geese, or even herons are all bad for ponds with fish, as they contribute to stress and also pollute the water with huge amounts of ammonia. Herons should be deterred regardless as they can cause injury to koi and will even take smaller pond fish if they can. Ducks and other animals may seem like a novelty when they first arrive but they’re actually a huge problem for ponds with koi or goldfish. Not only is their faeces (poop) a potent fertilizer packed with ammonia, but they carry a large range of diseases, bacteria, and parasites.

Ducks, the same as herons, should be deterred from landing in your pond at all costs if you want to maintain stable water quality and happy fish. This can be achieved with a mix of pond netting and specific deterrent methods, such as decoys, sprinklers, and sound systems. For more information on these topics, you can see our dedicated articles below:


5) Clean Bottom Sludge and Muck

Pond vacs make short work of sludge and bottom muck, and are a good choice for larger koi pond cleaning.

Bottom sludge is natural and doesn’t always need to be cleaned for ponds without fish, but should be carefully monitored if you have goldfish or koi carp. Sludge can become very dangerous if it builds up to high levels, as it lowers oxygen content and raises waste products in the water. Beneficial bacteria will constantly be breaking down sludge, but they can’t always keep up with demand. Ideally, if you have koi especially, sludge should be cleaned out at the end of autumn before winter and at the start of spring. This ensures the best possible hibernation period and start to the following year. Sludge can be removed with either natural bacteria based products or manual vacuums, although we find vacs much better suited for large ponds. For smaller ponds, it can simply be raked out with a regular pond net as a vacuum may be overkill.

Sludge also provides nutrients for nuisance algae, so if you also suffer with constant algae problems it may be beneficial to always keep sludge low throughout the year. If your pond has plants, having a small amount of sludge may be useful as the sludge will provide the roots with extra nutrients so they can compete with algae. Since potted plants generally have their own substrate and can also suck nutrients directly from the water, it’s up to you whether you clean all sludge or leave a little behind.


6) Carry out Regular Water Changes

Water changes will help remove ammonia, as well as replenish minerals and alkalinity.

If you have koi or goldfish, you should be carrying out regular water changes to help replenish trace minerals, alkalinity, and remove harmful substances. Contrary to common belief, water changes are not bad for ponds, and they should actually make up part of every fish keepers regular maintenance routine. If you’re currently suffering from high ammonia parameters, ask yourself this – when was the last time you carried out a large water change? If the answer to this is a “long time ago”, or “never”, start changing that water today!

Performing a weekly 10% water change will not only help remove a large chunk of free-swimming ammonia and help get your parameters down, but it will also replenish essential minerals and alkalinity. Minerals such as calcium and magnesium are important to fish health, in particular the regulation of salt within the blood, and they get these directly from the surrounding water (and food). Likewise, alkalinity is important to prevent sudden swings in pH and works as a ‘buffer’ to prevent this from happening. Alkalinity, or KH, comes in the form of carbonates, such as calcium carbonate, and needs to be replenished regularly as they become used up by acids (rain) and beneficial bacteria (yep, they love carbonates, too!). Check our out guide below on how to perform water changes in ponds for the best results:-


7) Invest in a Top Quality Fish Food (& don’t overfeed your fish!)

The amount of ammonia in a pond is directly related to the amount of fish waste, so keeping waste as low as possible is ideal. Good quality feeds will have lower ‘filler content’ and more ingredients fish can actually use, such as protein, fats, vitamins and minerals. Investing in a good quality fish food will reduce the amount of waste your fish produce, and also enhance the growth, color, and immune systems of koi and goldfish. Top feeds also source their protein from aquatic cultures, such as krill, anchovy, and fish meal, making them more optimal and more tasty – which fish will love! For more info about selecting the right feed and what ingredients to watch out for, see our full guide here.

Also it should be noted that you should not overfeed your fish as any left over food will simply sink and contribute to bottom muck. Since quality food is also high in protein and nutrients, any left around can cause issues with cloudy water and algae bloom. Keep an eye on your fish and make sure to monitor feeding to ensure they’re eating all their grub!

4 thoughts on “How to Lower Ammonia In Fish Ponds (And Keep it Low)

  1. I live in ontario canada and, we have fluctuations in temperature. I have a small pond (5’×8′ w depth 4′) out pump filter and water fall is off. We have a bubbler and de icer going. At the moment we have high amonia levels. I added salt and there was some improvement. My husband wants to skim the bottom of the pond where the five koi fish are hibernating. I’m worried this will make matters worse. Any suggestions or thoughts? We are going to do a partial water change too. It is still too cold to add our pump and filter.
    Regards Niamh

    • Hi Niamh,

      What’s giving you the impression of high ammonia? Have you performed a water quality test? Or are fish becoming sick?

      Cleaning the bottom of the pond in winter, especially without filtration, isn’t recommended as you’ll actively stir up all the nasty substances that have been decomposing slowly over the season. In warmer seasons the koi would tolerate it, but as they’re in hibernation, it will place a lot more stress on them and likely only increase the amount of free-swimming ammonia in the water.

      Water changes are indeed the best way to go in your situation. Personally, I’d start with a 20% water change and test for ammonia again the following day. If ammonia is still high, perform another water change and re-test. As soon as ammonia has been sufficiently reduced, a 10% water change weekly will likely keep levels safe until spring cleaning starts.

      Adding fresh water through water changes will also help replenish pond alkalinity/KH, which is very important to prevent dangerous pH fluctuations. Beneficial bacteria also require good alkalinity (carbonate) levels so they can continue to break down waste, even in winter.

      Hopefully the water changes do the trick. Let me know how things turn out!

  2. Hi I have a very high ammonia level have done 2 50% water changes and it still high I’ve lost 6 koi and I am worried I will lose all my other big koi can someone help me plz

    • Hi Paul,

      I’m really sorry to hear that! Nothing is worse in this hobby than losing fish.

      Could I ask where are you getting your water from? Is this your mains supply or from a natural source, such as a well?

      If ammonia is still testing high after 2x 50% water changes, the ammonia may be in the actual water supply itself. You can determine this by directly testing the mains supply with a water quality kit (same as the pond kits) or contacting your water company to confirm.

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