Goldfish in a Koi Pond

Goldfish in a koi pond can be a much welcome pop of color in your backyard water garden, but it’s important to introduce these living things to your pond in the correct way in order to avoid any of them becoming sick or dying from the transfer. Koi ponds are delicate ecosystems that have to be adjusted carefully when living things—either animals or plants—are added into the mix. 

So how do you add goldfish to a koi pond? Goldfish should only be put in a koi pond that is planted and has had several months to mature. This allows it to build up appropriate levels of ammonia-reducing bacteria and other microbes necessary for a healthy aquatic environment. Once the water is at suitable levels to sustain life, goldfish can be added. 

Goldfish In A Pond Image

Goldfish and other aquatic animals are one of the greatest rewards of setting up a koi pond, but it’s important to know how to care for them properly to get the most out of them. Read on to find out more about keeping goldfish outdoors and how they can be successfully integrated into a koi pond. 

How to Introduce Goldfish to a Pond

When you’re getting ready to add goldfish to your outdoor koi pond, keep in mind that the environment of the koi pond has to be fully matured before adding any kind of aquatic life that is dependent on the chemical composition of the water. Fish and other pond animals are dependent on having the appropriate levels of oxygen, nitrates, nitrites, ammonia, and other trace minerals in the water at any given time for optimal health. 

The single most significant thing that must be done to a koi pond to prepare it for goldfish is to establish the nitrogen cycle, same as you would with an indoor aquarium. Establishing the nitrogen cycle means allowing a colony of beneficial bacteria to build up in the water that strips the water of poisonous waste toxins generated by living things (ammonia) and converting it to other harmless compounds.

This nitrogen cycle is present in any water that contains live aquatic animals like fish, all of whom are dependent on it in order to live. Captive fish are especially dependent on established nitrogen cycles because unlike wild fish, they are contained in a closed ecosystem. If there is no nitrogen cycle established to convert their wastes, the water will quickly turn bad and kill anything living in it.  

The Nitrogen Cycle Helps Goldfish In A Koi Pond Live
The Nitrogen Cycle Helps Goldfish In A Koi Pond Live

To establish a nitrogen cycle in a koi pond in preparation for adding goldfish, add plants and then run the pond continuously for a few months. The bacteria from the plants and their detritus becomes trapped in the koi pond. It then becomes the foundation for the colonies of beneficial bacteria that are necessary for neutralizing poisonous ammonia in closed water systems like koi ponds.  

Selecting Goldfish for Your Koi Pond

There are a few things to take into consideration when deciding which goldfish to select for your koi pond. If you have indoor aquariums, a koi pond can be a great place to “retire” indoor goldfish who are becoming too large to fit comfortably into an indoor tank (because of their large adult size, goldfish should ideally not be kept in any tank smaller than 30 gallons). 

Goldfish are a very social fish that prefer to exist in a group, so it is better to get several goldfish and introduce them at once rather than just introduce a single fish by itself. While a lone fish in a koi pond might look idyllic and beautiful, the fish is likely to be unhappy in this kind of setup. Goldfish should be kept with at least one other of their own kind, and preferably more. 

Fancy ponderous goldfish such as pearlscales and black moors tend to have mobility problems and are especially vulnerable to predation when kept outdoors. Instead, fancy goldfish breeds that emulate the quick, bullet-like body of the koi, such as shubunkins or comets, are a much better choice for outfitting your koi pond. 

Goldfish are a good choice for smaller koi ponds that aren’t large enough to accommodate full-grown koi. But on the flip side of this, goldfish should not be introduced to a koi pond when they’re too small. The smaller ornamental pond fish are, the more vulnerable they are to predation by everything from raccoons and herons to neighborhood cats. 

Other Fish That Can Be Kept with Goldfish in a Koi Pond

While other fish can technically be kept with goldfish in a garden pond, there are some caveats to that scenario. Here are some of the issues that crop up when people keep goldfish with other fish in a pond:

  • Keeping cold water minnows with goldfish: Another popular choice of a companion fish for goldfish other than koi are cold water minnows such as white cloud mountain minnows. While these minnows do well in unheated koi ponds and can add some much-needed activity to your pond, minnows can also have a bad habit of chasing and harassing goldfish, especially those with long fins.
  • Algae eaters and goldfish: Algae eaters are often put into ponds with goldfish and in many cases they can cohabitate peacefully. But some algae eaters can develop a bad habit of latching on to goldfish while they sleep near the bottom of a pond or fish tank and eating the slime coating off their skin. This can leave goldfish vulnerable to ulcers and other health problems. 

Algae Eaters And Goldfish

Many fish can be kept with goldfish provided the fish are introduced or kept together while small/young and are kept in a koi pond which has plenty of resources for all of the animals present in it. However, behavior of pond fish needs to be observed to make sure that the goldfish aren’t being harassed by other fish in the pond (or vice versa). 

Do Koi Eat Goldfish? 

Keeping goldfish with koi can be done successfully. In larger ponds, these fish are often combined. However, in smaller ponds without a lot of cover for fish to hide in, goldfish can be quickly outgrown by their koi neighbors, and koi—like goldfish—can be notorious for eating anything they can fit in their mouths. That includes any goldfish that are small or don’t have the space to escape larger fish.

Other Aquatic Animals That Can Be Kept with Goldfish

Along with fish, there are also different kinds of other invertebrates and other aquatic animals that can be kept in a koi pond with goldfish. Some of these animals are very good goldfish companions, while others can be problematic depending on the circumstances. Here are some of the choices available: 

  • Snails: Snails are a great companion for goldfish, as they are completely harmless to the goldfish and are slow-moving enough that goldfish tend to leave them alone in return. A popular species of snail in koi ponds is the Japanese trapdoor snail, which is prized for its ability to survive cold winter temperatures well.
  • Frogs: Frogs are a fun addition to your goldfish pond and have the added benefit of helping to keep down pesky insect populations that can congregate around outdoor ponds, including mosquitoes. It’s important if you do decide to add frogs to your outdoor pond that you choose a species that is considered non-invasive to your area.
  • Turtles: Turtles can sometimes be kept harmoniously in a koi pond along with goldfish, provided the goldfish are large enough and the turtles are small enough. However, if the goldfish are too small and the turtles are too large, the turtles will often eat whatever goldfish are small and slow enough for them to catch.
  • Crayfish: Crayfish can be kept in a koi pond with the same caveat as the turtles—depending on the personality of individual crayfish, they may try to go after any goldfish that they deem small enough to consider prey, and even with larger goldfish they can sometimes be prone to trying to grab fish with their pincers, inflicting injury. However, with plenty of food, crayfish can sometimes be kept peacefully with goldfish in a big pond with plenty of hiding places for both. 

The larger a koi pond is, the easier it is to keep multiple species together peacefully without running into conflicts between them. This affords each animal in the pond more resources and also allows them the space to stay out of each other’s way. 

To avoid having to micromanage your koi pond’s community, the best option is to stick with goldfish only. Not only does this prevent you from having any kind of issues with in-fighting between your different pond populations, it also gives you more space to keep multiple goldfish with a smaller bioload in the pond. 

Best Outdoor Plants for Goldfish

For the best outdoor pond, you’re going to need to integrate some live plants, both aquatic plants and marginal plants—those that exist along the edges of the pond. 

While goldfish can tend to be a bit destructive towards live plants because they love to go rooting around on the bottom of the pond, there are some plants such as water lilies which can do very well in a goldfish setup. 

Here are some plants that are good for koi ponds with goldfish: 

  • Duckweed
  • Anacharis
  • Anubias
  • Water lettuce
  • Hornwort

Plants can be a great way to add an additional source of natural food for goldfish as well as shelter for them to hide from predators. Because goldfish tend to graze on the live plants in their pond or tank, it’s a good idea to overplant and add more plants than you would think was necessary. 

It’s also important to rinse plants thoroughly before adding them to the pond to avoid introducing either microbial diseases or unwanted snails and other invertebrates to your pond system. While some snails are wanted in the koi pond, others such as bladder snails can quickly overrun your pond system if left unchecked and can even necessitate the addition of “assassin” snails to clean up the population. 

Goldfish Predators in Koi Ponds

One of the biggest challenges to introducing goldfish, koi, and other ornamental pond fish outdoors is having to deal with predators. Loose outdoor cats can be a problem, as they will sometimes catch goldfish out of a pond for sport rather than food, but wildlife such as raccoons, herons, and other birds can also wreak havoc on your pond goldfish populations. 

A Predator Of Goldfish-The Heron

There are a few measures that aquarists can take to help prevent their fish from being taken, however. Here are some tips for making sure that predators don’t eat all of the goldfish out of your pond: 

  • Install an arbor or gazebo over the pond. Not only can a structure like this help prevent unwanted dead leaves from blowing into the pond when the weather starts to turn, they can also help prevent birds such as herons that are flying overhead from seeing your pond. Once a heron knows of a food source such as a fishpond, they will fish it until the fish are all gone.
  • Build a pond with steep sides. Deep ponds not only allow fish to sink to the bottom of the pond to escape any predators that are fishing from the surface, they also prevent predators from wading in after fish, since animals like raccoons are more likely to enter water with a sloping side versus a steep one. Keep in mind that a pond with steep sides is more likely to accidentally trap and drown small animals, however.
  • Build a fence. Building a fence—especially if you build a fence around a deeper koi pond—can help you avoid predation from shallow water predators such as racoons and herons. Herons can be a serious problem for outdoor fish keepers since they are typically protected animals and are difficult to drive off once they’ve found a source of food. Raccoons will go after goldfish in a pond but prefer a shallow pool to a deep pond where the fish are not as easily visible. 

While there are steps you can take to protect your outdoor goldfish , the fact of the matter is that when you keep fish outside, you’re probably going to be dealing with some losses to predators as well as environmental fluctuations, so plan on adding a few fish later down the road. 

Large predators of ornamental pond fish can be difficult to discourage, but many problems with smaller pests such as insects and microbial diseases can be avoided through the proper maintenance of the koi pond’s water chemistry. 

Adding More Goldfish to Your Koi Pond

Eventually, after you’ve established a koi pond with goldfish for several months or years, you might find yourself wanting to add additional fish in with the goldfish you already have. If you plan on adding more goldfish to your koi pond, here are some things you need to consider. 

  • Bioload: How many goldfish do you currently have in your koi pond, and how big is it? When fishkeeping, it is always better to keep less fish in a tank or pond than the water’s maximum capacity, as this provides a buffer that prevents sudden drops in water quality. If you already have a pond brimming with goldfish, you might want to reconsider adding more, as this could potentially cause water chemistry problems.
  • Quarantine: When adding additional fish to an established koi pond, they should be kept in a quarantine tank for several days to verify that the fish aren’t ill and don’t have any issues with parasites. It’s very important to take this step, since introducing a sick fish into a closed system like a koi pond could potentially wipe out your entire stock of pond fish. 
  • Size: If a koi pond already contains very large goldfish, it’s not a great idea to add very small goldfish into the pond with them. While goldfish mostly eat aquatic plants, they are omnivorous, and a large goldfish or koi will sometimes eat a juvenile goldfish if it’s small enough to fit in the larger fish’s mouth. 

Goldfish are relatively easygoing community fish and you can often add additional fish after the fact without having any problems, but it’s important to think ahead before purchasing additional fish to make sure that your pond is prepared to take them on. 

Goldfish Are a Great Choice for Koi Ponds

Cartoon Image for Goldfish In A Koi Pond Post

Many people may think of bowls when they think of ideal homes for goldfish, but the reality couldn’t be further from the truth. Goldfish require much larger habitats than many amateur fishkeepers provide for them, and as a result they don’t live as long as they could. In fact, if kept in koi ponds, goldfish are able to survive for decades and some of them can grow just as large as a koi. 

When a few minor precautions are taken to make sure that the goldfish have a proper environment when they’re introduced to the pond, investing in backyard goldfish can provide a gardener or homeowner with viewing pleasure for years to come.

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4 thoughts on “Goldfish in a Koi Pond”

  1. Great point in saying that goldfish in a koi pond can add some to a backyard water garden. I was not aware that several months are needed so that the pond can mature before adding fish. Algae eaters are fish that I would definitely have considered adding to the pond, so it’s good to know that they can be harmful to goldfish in that they may latch on while the fish is swimming and cause ulcers. This is something to keep an eye on thanks to your information here.

    In regard to other aquatic animals, outdoor plants and predators, you have provided some extremely helpful and extensive information. As one who has been considering a goldfish pond for some time now, I am thankful to have come across your post, which I will save to reference when the time soon comes for a koi pond. Great work!

    • I’m glad I planted a vivid picture of the pros and cons of mixing goldfish in an outdoor pond. Some fish will go about peacefully with goldfish and others would tear them apart. For example, I don’t want minnows chasing around other fish who have those tempting(apparently) long tails, so I probably wouldn’t have them at all. You could try to put a barrier of some kind between some fish so they have their own little world free of pesky other fish. I am glad this inspired another future koi pond owner.

  2. Hello there,thanks for sharing this very awesome article i know it would be of great help to the public as it has been of help to me….my goldfish’s home got broken and since i cant other anything i have to put her in a bowl.

    I never thought that a goldfish could saty in a koi pond;although they are all fish but i was some hoe as someone has already told me about it….i would follow your tips and try them out.

    • Hi David

      How are things going with you? A lot of people aren’t aware of the possibility of stocking goldfish in a pond. I had a few goldfish in bowls when I was a kid. We tried the best we knew to take care of them. This included changing out the water on a weekly basis and feeding them daily. 

      The problem is they kept jumping out of the bowl despite our extensive fish care routine. I use that word “extensive” very lightly. We didn’t know cool bits of information such as to add water conditioner to our water before change out, to clean your bowl with warm water and non-iodized salt, or making that condition stand for an hour in your water before adding back in your fish.

      “Knowledge Is Power” is really true. Looking back on the few fish deaths I experienced at a young age I can’t help but wonder if the fish weren’t suffocating or poisoned from the chlorine in the water. Hindsight is 20/20. I’m glad to inform and will be happy to see you following my tips for adding goldfish to a koi pond.


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