6 Reasons Your Pond Fish are Dying


Keeping pond fish alive isn’t about having the right amount of luck. After all, koi themselves symbolize good fortune. But if you’re wondering why your pond fish are dying, you may need to dig a little bit. There are a few common factors that are behind most pond fish deaths.

Here are 6 common reasons why pond fish die: 

1. Poor Water Quality

Irregular water maintenance may not seem like a deadly killer, but poor maintenance routines can create toxic pond environments very quickly, particularly in new ponds. Regular water maintenance is necessary for maintaining a balanced ecosystem. As your koi and goldfish eat, they produce waste, or more importantly, ammonia. Ammonia is extremely toxic for fish and if ammonia levels get too high, your fish will die. The recommended ammonia level in your pond is 0 ppm. That means even a little setback in your early cleaning routine can be the reason your pond fish are dying.


Symptoms of ammonia poisoning in pond fish:

  • Reddening of your fish’s skin and fins
  • Damage to gills
  • Clamped fins
  • Irregular movement
  • Reduced slime coat, which puts your fish at high risk for parasitic or bacterial infections

What to do if your fish have ammonia poisoning

If you notice some symptoms of ammonia poisoning in your fish or if a water test indicates high ammonia levels, it is important to act quickly. If you start noticing some symptoms of ammonia poisoning, it is possible to save your pond fish before it is too late:

  1. Do a water change of 20% of your pond water. If your ammonia levels are still high a day later, you can do another 20% change. It can be harmful to change more than 30% of your pond’s water at any time.
  2. Add beneficial bacteria to replenish lost bacteria from the water change, but also to help reduce ammonia levels for the future. Beneficial bacteria is essential for creating a balanced ecosystem and can be useful for every pond.
  3. Add an ammonia remover to detoxify ammonia immediately. While your beneficial bacteria will do this eventually, this can give them a head start.
  4. If your fish are recovering from ammonia poisoning, you’ll need to protect them from diseases or infections until their immune systems are restored. Melafix is a gentle, non-medicinal solution that can be used if your fish have open wounds or sores. You should also add non-iodized aquarium or pond salt to help replenish your fish’ slime coat and choke out any parasites.

Ammonia poisoning is a common, but not inevitable part of pond ownership. There are a few things that you can do to prevent ammonia spikes in your pond.

How to prevent ammonia spikes in your pond:

  • DO refill any water loss due to evaporation.
  • DO regularly add beneficial bacteria to your pond.
  • DO use chemical-free, safe cleaners in your pond.
  • DO remove leaves, sticks, and other debris as soon as possible from your pond. When these decay, they also contribute to ammonia levels
  • DO test your water weekly for ammonia.
  • DO consider adding a filter compartment with bio-balls to encourage beneficial bacteria to colonize in your pond.
  • DO stick with it. As your pond matures, it should self-regulate. Toward the end of the pond season, your pond should need much less intervention (for ammonia, anyhow).
  • DON’T habitually scrub rocks and equipment. These harbour beneficial bacteria.
  • DON’T overstock your fish pond. Too many fish = more waste = more ammonia. Your pond will be easier to balance if you stay well-below your pond’s maximum bioload. As such, larger ponds tend to have fewer problems with ammonia spikes.
  • DON’T do extreme water changes (unless you have an ammonia spike). Pond water contains ammonia, but it also contains good bacteria that helps to balance your pond. Removing too much of these good bacteria through heavy water changes can cause ammonia levels to creep up again. It is still recommended to do weekly 10-15% water changes weekly

2. Oxygen Depletion 


Dissolved oxygen is essential for your goldfish and koi. Your pond establishes adequate levels of dissolved oxygen through proper aeration. Poor aeration can be one the quickest and most silent killers of your fish. Even if you have had an established pond for years, your koi will require more oxygen as they grow bigger. Overnight your koi or fish may die with absolutely no notice or symptoms. For your koi to thrive, it is recommended that your pond contains 7-9 mg/L of dissolved oxygen.

Signs of oxygen deprivation (hypoxia) in fish:

  • fish gasping for air at the surface of the water
  • fish remaining at the surface of the water, where dissolved oxygen levels are highest
  • sudden reduction in your fish’ appetite 

A couple factors that contribute to oxygen depletion are:

  • over population (or growing fish) in your pond
  • an algae bloom
  • aerobic bacteria
  • a heat wave in the weather (since warmer water cannot hold as much oxygen, a surge in temperature can have a detrimental effect on dissolved oxygen)

What do do if your pond lacks oxygen

As always, the best solution for this problem is prevention. But that doesn’t help you if your fish are already low on oxygen. There are a few ways you can increase aeration and oxygen in your pond:

  • If you have a waterfall or fountain, you will most likely have enough aeration depending on the flow-rate and the depth of your pond. Increasing flow-rate by increasing the size of your pump or increasing the angle of your waterfall can increase aeration.
  • If you are concerned about algae blooms, you can add another spitter or something else to break your pond’s surface and increase dissolved oxygen, but make sure you are not overtaxing your pump.
  • If your pond is deeper that 48”, it is recommended you use an airstone to supplement oxygen levels.

3. Chlorine and Chloramine 

Though tap water may be safe for humans to consume, it is often toxic to aquatic life. Particularly in Canada, chlorine and chloramine are commonly used in water treatment plants to disinfect tap water. Unfortunately, chlorine and chloramine are both extremely toxic to fish. Both toxins will attack your fish’s gills, preventing them from breathing.

If your goldfish or koi come in contact with chlorine or chloramine, they could die quickly and without any evidence. It does not matter if you are simply topping off your pond, the smallest amount of chlorine or chloramine can affect your fish.

What is especially worrying about chloramine is that it does not dissipate over time as chlorine does. This means that every time you add more tap water to your fish pond or aquarium, more toxic chloramine accumulates.

Therefore, the best solution to prevent death by chlorine or chloramine is to use a water conditioner or dechlorinator every time you add new water to your pond. Irrigation (or well) water or rain water, which does not go through water treatment, can also be used. However, these water sources can still contain higher than normal levels of nitrates, phosphates, or other chemicals which can have unwanted effects in your pond. And no matter what your water source, you are safest to use a product to detoxify heavy metals. 

Here are some symptoms of chlorine and chloramine toxicity:

  • Fish are gasping for air
  • Fish dying very rapidly and without symptoms or notice

Solutions to prevent pond fish death due to chlorine and chloramine poisoning:

  • Always use a water conditioner or dechlorinator that removes chlorine or chloramine (depending on what your local water treatment plant uses)
  • Make sure that you follow directions and use the appropriate amount of product for removing either chlorine or chloramine as it often differs.
  • Use activated carbon in your filter to absorb chlorine, chloramine, and other impurities. If you do not have a filter, you can drop in a bag of activated carbon in an area that gets plenty of water circulation (i.e. in the path of a waterfall or filter). Replace activated carbon every 4-6 weeks.
  • Use irrigation water, but with careful testing for common runoff contaminants such as nitrates and phosphates.

4. Fish Jumping

It can be very unsettling to look into your pond and find a precious koi or goldfish missing. First, check the perimeter of your pond. Koi have a notorious reputation for jumping out of fish ponds. Hopefully, it will have been a recent jump and your fish will pull through. If your fish is missing, it  may have fallen prey to a predator.

If your koi are jumping ship, the first thing you want to do is check your water quality. If you have poor water quality, your fish may not be happy with the environment they are in. That alone can be reason for your fish to attempt to jump. Your fish pond may have high ammonia levels, low oxygen levels, or chlorine/chloramine toxicity (see above for solutions to these problems). Fish can also jump to escape predators, unwanted suitors during mating season, parasites or infections, and for fun.

Solutions to prevent fish death from goldfish or koi jumping out of your pond:

  • Frequent water testing for toxic ammonia, nitrates, and pH.
  • Monitor oxygen levels and check your fish for parasites, diseases, infection, or other signs of stress, such as predators.
  • After you have ensured your pond’s water quality and the health of your fish, pond netting over your pond at night or any other time you will be away from your fish for long periods of time will prevent fish from jumping out of your pond. Pond netting will also prevent predators from accessing your fish.

5. Predators


You may rarely see content fish in your backyard pond, particularly if you have a large pond with many hiding spots. Pond fish such as goldfish or koi are naturally timid and often spend most of their day hidden from sight. Try feeding your fish a tasty treat, such as citrus slices (though be sure to remove any excess after an hour) to get them to surface. If your fish are missing, a raccoon, heron, or some other neighborhood pest (even wandering dogs or cats) could be to blame.

There are a few ways you can prevent fish death due to predators. First of all, make sure your pond includes hiding places for your fish. Natural hiding places such as water plants or rocks, or artificial hiding places will suffice. Water plants have many other benefits for your pond, so are an optimal choice. Decoys, which are artificial predators, are often enough to convince most real predators, which are solitary hunters, to move on to less contested waters. However, these are most effective if you know what type of predator is hunting your fish (i.e. heron decoy for herons). Adding pond netting to your pond can help control fish death due to predators, but this only works as long as the netting is on your pond.

Not all predators come from outside of your pond. Make sure that you have not overstocked your koi pond. Overstocking can cause your fish to become territorial and turn on each other. You may also want to increase the fish food you are feeding, or start supplementing food if you are not feeding (larger ponds often have natural sources of food for koi and other fish).

Here are some solutions to prevent pond fish death caused by predators:

  • Increasing natural or artificial hiding places with the additional of plants, rocks, or something else.
  • Use of a predator decoy
  • Pond netting
  • If you think your fish may be turning on each other, consider decreasing the amount of fish in your pond or increasing available food.

6. Disease or Infection

There are many ways your koi or goldfish can contract a disease or infection. Just as with our own environment, there are pathogens everywhere. Most of the time, healthy fish should be able to fend off infection. Problems usually arise when a fish’s immune system is compromised, such as when the fish is under stress.

Here are a few factors that can make your fish more susceptible to infection:

  • Any stress, including stress from poor water conditions, fish fighting (often from too many fish in too little space), scarcity of food, introduction of new fish
  • If your fish has wounds, either from an abrasion (sharp rocks or edges from broken equipment) or wounds endured from other fish while fighting
  • A sick newly introduced fish

There are four types of pond fish infections with various symptoms:

  • Parasitic Infections - These infections can be internal or external. Internal infections can sometimes be difficult to diagnose as they can have few external symptoms. External parasitic infections can sometimes appear as raised portions or growths, a whitish skin coating, or visible parasites on your fish’s scales or body, but can vary in colour and appearance.
  • Bacterial Infections - Bacterial infections in pond fish have varying symptoms, but can often appear as red or swollen patches on your fish’s scales or body, open ulcers or wounds, bulging eyes, tearing of the fins or the mouth, and other symptoms.
  • Fungal Infections - In pond fish, fungal infections are easily characterized by white, cottony patches on the fish body or scales.
  • Viral Infections - Like bacterial infections, pond fish viruses have many different symptoms, depending on the virus and can range from harmless to deadly.

Your first course of action when you notice unusual symptoms in your fish should be to check your water quality. Many diseases and infections are brought about solely by poor water quality. Disease or infections are often triggered by stress. Stress weakens your fish’s immune system, making the fish susceptible to disease or infection.

Depending on the infection and how quickly you notice the infection, there still may be time to save your fish. Quarantining affected fish in a separate aquarium and subsequently treating them according to their affliction is the best way to prevent the infection from spreading. Protect your remaining fish and those who may have the infection, but show no symptoms, by treating your entire pond as well. Whenever possible, use natural treatments such as Melafix in favour of antibiotics for less system disruption.

Keep your fish healthy and happy by closely monitoring water conditions. Neutralize toxins quickly and take care of water problems before they have a negative effect on your fish’s stress levels. Run regular maintenance in your pond, including partial water changes monthly or when needed, and make sure water is circulating sufficiently for oxygenation. Also quarantine all koi or goldfish before introducing them to your pond. For carefree disease control, install a UV sterilizer to eliminate free floating pathogens. Your water will be sterile for your fish and also free of floating green algae.

Here are some symptoms for disease or infection:

  • Any abnormal growth, wounds, colouration, or other physical attribute on your koi or goldfish
  • Unusual behavior or flashing
  • Irregular eating or decrease in activity

Here are some solutions for diseases:

  • Maintain good water quality and keep up with regular maintenance and water testing
  • Keep in mind your pond’s biocapacity, or the maximum amount of fish that can be housed comfortably without additional stress
  • Quarantine new koi or goldfish before introducing them into your pond
  • Quarantine fish at the first sign of infection or disease
  • Install UV sterilizer to eliminate free floating pathogens
  • Use smooth river rocks or boulders in your pond to prevent cuts or wounds
  • Try non-iodized salt (also called aquarium salt) to restore your fish’s natural slime coat, thereby reducing stress


While it might be easier to blame it on bad luck, your pond fish are dying for a reason. But the good thing is that (nearly) all of these common koi and goldfish deaths are preventable and treatable with the right plant of action. Your best bet is to keep up with your pond maintenance, test your water regularly, and observe, observe, observe. The quicker you uncover and remedy what is killing your pond fish, the sooner you can have a healthy, beautiful, and problem-free pond.

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