Can You Put Koi Fish with Oscars?

After successfully establishing your very own koi pond, you may decide you’d like to give your pet koi fish some company, in the form of introducing other fish into their environment.

When putting different species of fish together in one enclosure, it’s important to make sure the two species are compatible with one another.

Different species of fish have different environmental requirements, as well as drastically different temperaments.

Just because two species of fish look beautiful swimming together, doesn’t necessarily mean they belong together. Here, we will aim to answer the question: can you put pet koi fish with Oscars?

Why Oscars and Koi Are Incompatible?

Koi fish and Oscars are two of the most highly sought-after species of pet fish by casual keepers and experts alike.

Both species are beautiful in their way, not to mention intelligent. However, the reality of the matter is that Oscars and koi do not belong in the same tank, for several reasons.

Size Differences 

The average Oscar can grow to be up to 16inches long, this is quite big for most aquarium fish.

For just one Oscar, you’d need a tank with a capacity of about 55 gallons. Koi grow even larger than Oscars, reaching a whopping 36inches, that’s more than twice the size of the average Oscar.

This means that to adequately keep an Oscar and a Koi in the same space, you would need a tank that is well over 100 gallons in size.

Not only would this be difficult to keep in your home, but it would also be exorbitantly expensive to maintain, for just too fish.

Not to mention, a tank isn’t the ideal enclosure for a koi in the first place. Koi fish belong in outdoor ponds, an environment that is less than ideal for Oscars.

Water Requirements

Different species of fish thrive in different temperatures of water, the same can be said for koi and Oscars.

For Oscars, the perfect water temperature ranges between 74- and 80 degrees Fahrenheit.

On the other hand, the koi originated in the frigid waters of Japan, where they would rest under the frost during the winter months.

The ideal temperature for koi is between 65- and 75 degrees Fahrenheit.

What this means is that it will be next to impossible to keep both fish satisfied with the temperature of their tank at all times.

Differing Personalities

Oscars are known to be one of the more aggressive species of pet fish, not to mention highly territorial.

These fish have been known to attack and even kill fish smaller than themselves, which means younger koi sharing their enclosure could end up being killed by the larger Oscar.

Even if the Oscar can’t kill a koi larger than itself, it won’t be afraid of bullying it. On the other hand, koi are known to be docile creatures.

They have mild temperaments, leaving them unable to defend themselves against the more aggressive koi.

This is one of the primary reasons that expert fish keepers warn against allowing koi and Oscars to share an enclosure.

You can get the size of the tank and its environmental conditions just perfect, and there still would be no way to prevent the Oscar from attacking and potentially killing your pet koi fish.

Growth Rates

You may think that if you get an Oscar and a Koi as babies, the two fish will grow up side by side, and become accustomed to each other.

You may think this would prevent Oscar from attacking the koi, but this is wrong.

You see, Oscars tend to grow at around an inch per month, reaching their maximum size in just over a year.

Koi fish grow at almost the same pace for 6 months, before their growth rate slows down.

Although the koi will eventually grow much larger than the Oscar, the Oscar will reach its maximum size first, giving it an advantage over the more docile koi, and allowing it to bully it.

What Can Go Wrong When Keeping Koi and Oscars?

If you’re still considering keeping your pet koi fish and an Oscar in the same enclosure, it’s important that you understand the worst-case scenario.

A fully grown Oscar will not hesitate to devour a younger koi.

Even larger koi will still get bullied by Oscar, and most likely killed.

Experienced fish keepers understand these potential consequences, and so choose to avoid them.

Can You Crossbreed Koi and Oscars?

In recent years, hybrid specimens produced by crossbreeding two separate species of fish have become increasingly popular with raisers.

Some of the most common include crossbreeding koi with goldfish, or with mirror carp to produce ghost koi.

This raises the question, can you crossbreed koi and Oscars? The simple answer is no, you can’t.

The various breeds of Oscar can only be crossbred with other breeds of Oscars.

Can Oscars Live Alone?

Like most other species of fish, Oscars enjoy the company of other pond mates.

However, if certain living conditions are met, and the quality of its aquarium is maintained, your Oscar will be happy living on its own.

This is arguably a much better alternative to keeping An Oscar with a koi.

So, if you already have a koi pond, but do still want to raise an Oscar, consider building it a separate aquarium, where its living conditions will be better suited to its health and well-being.

How To Care for Your Oscar

If you’ve accustomed to raising koi fish, but want to get your first Oscar, there are a few things you’ll need to learn.

Here’s how to properly care for your Oscar.

Tank Size

As mentioned previously, Oscars grow very quickly, reaching around 16-20 inches in under two years.

You may think you can save money by starting with a smaller tank, and then investing in a larger one down the line, once your Oscar gets bigger.

This is a mistake. Not only will your Oscar be unhappy, but you’ll also actually end up wasting money in the long run.

Water Conditions

We previously talked about the difference in water preferences between koi and Oscars.

As tropical freshwater fish, Oscars thrive in warmer waters, ranging between 75 and 80 degrees Fahrenheit, with a pH level between 6 and 8.


The dietary habits of the Oscars and koi are also very different. Oscars enjoy eating live worms and other insects.

However, you have to ensure that you source live food from a reliable source, as they may contain pathogens that can be harmful to your pet Oscar.

The safest way to feed your Oscar is with frozen food to ensure that it is free of pathogens. Unlike koi, Oscars are carnivorous fish.

This means their primary diet consists of worms and insects, although they have been known to enjoy fruits and vegetables, on occasion.


If you’ve been raising koi for a while, then it should come as no surprise to you that fish are intelligent and curious creatures that crave regular mental stimulation.

There are several ways to keep your pet Oscar entertained, from sinkable toys to ping pong balls.

If you place a mirror in your Oscar’s tank you might even get to see it interacting with its reflection, believing it to be another fish.

What Fish Can Live with Oscars?

If you would still like to house your Oscar with other fish, some species make better tank mates than koi.

The best fish to keep in the same tank as an Oscar include Blue Acara, Catfish, certain breeds of Cichlids, and Silver Dollar Fish.

However, the ideal fish to keep with your pet Oscar is another Oscar. Generally, you should avoid keeping two emails in the same tank.

A male and female Oscar will get along just fine, as will two female Oscars.

As we discussed previously, though, Oscars grow very large, and any fish you decide to keep in the same tank will require more space.

If you have a tank exceeding 100 gallons, keeping an Oscar and another larger fish, such as a catfish, shouldn’t be an issue.

But if you don’t have the means to support such a large tank, we recommend sticking to keeping a single Oscar in a 55-gallon one.

What Do You Think?

Now that you’ve read this article on the dangers of keeping koi and Oscars in the same tank, what do you think?

Are you an experienced koi owner looking to raise your first Oscar? If so, we hope this guide provided some useful insight into what you should avoid.

Or, are you interested in raising koi for the very first time? Koi raising is an immense responsibility, and we recommend learning everything you can before investing in your own koi pond. Start by dedicating yourself exclusively to koi.

Down the line, if you’d like to expand your collection, you can do so. But first, you must learn to provide your pet koi fish with the care and comfort that they deserve.

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