Can Oscar Fish and Koi Live Together?

Both Oscars and Koi fish are outstanding specimens with particularities that make them unique in the aquarium world. It’s understandable why so many fish owners try to pair the 2 and accommodate them in the same tank.

But can they live together? Should you place them in the same environment and, if so, what should you expect?

Today’s article will look into the world of Oscar fish and Koi to explain why these 2 may not be such compatible tank mates. Unfortunately.

Should You Keep Oscar Fish and Koi Together?

No, you should not keep koi carp and oscar fish together, because they are not compatible. due to their huge differences in tank size requirements, water requirements, temperament, behavior and diet. Oscars are aggressive and they grow really fast, becoming a danger for even bigger koi carp.

Reasons to Avoid Keeping Oscar Fish with Koi

These beautiful species are, unfortunately, incompatible for several reasons.

– Their Size

Oscars can grow quite large, between 8 to 16 inches. This means that you will require a 50-gallon for one Oscar alone. This is already a huge investment and a lot of space occupied by one fish.

Now consider the Koi. This fish can grow to an astounding 36 inches in length, more than double the Oscar. Consequently, you will require a 100-gallon tank to keep one Koi alone.

There is no way to find a tank fit for both an Oscar and a Koi. We’re not even going to mention the possibility of holding multiple Oscars and Koi fish in the same environment.

And the fish’s size is but the smallest of your problems.

– Water Requirements

This is another area where the 2 species vary drastically, more specifically, water temperature. Oscars prefer water conditions similar to that of guppies. Their favorite water temperature ranges between 74 to 80 °F, with any significant variations outside these limits causing the Oscars visible discomfort.

The Koi, on the other hand, prefers colder temperatures. In nature, the Koi may even hibernate under ice, so long as the water is deep enough. In captivity, the Koi prefers water temperatures between 65 to 75 °F. Their maximum required temperature is the minimum required temperature for Oscars.

In other words, you cannot accommodate one species without making the other uncomfortable.

– Oscars’ Aggressive Nature

Oscars are intelligent, curious, and, most importantly, aggressive and territorial. This makes them unfit for living with the peaceful Koi. Oscars may exhibit abrupt territorial behavior, attacking anything in sight, especially smaller and weaker fish.

Oscars also tend to hunt and eat everything smaller than them. They make no difference between their regular prey and tank mates that they shouldn’t eat. This makes them a threat to Koi, especially in their early lives, when they’re smaller and more vulnerable.

Once the Koi reaches the same size as the Oscar, the latter will no longer hunt and kill it. But that doesn’t mean bullying is no longer an option. Oscars are known bullies, especially when the water conditions are less than ideal. Oscars will become notoriously more aggressive in poor water conditions, which can spell disaster for the tank dynamics.

– Oscars Grow Very Fast

Oscars grow faster than the Koi, which will create a size imbalance fast. This will lead Oscar to kill the Koi for food, especially since Oscars consume more live food than other fish species.

The Oscars’ notorious growth rate makes them unfit for cohabiting with many fish species, especially those who grow slower. This is why so many Oscar owners prefer to keep Oscars alone and not have to worry about aggression, bullying, and killing.

As you can see, Oscars and Koi are in no way compatible tank mates. If anything, water temperature differences alone will break their partnership before it even begins.

Can Oscar Fish Live Alone?

Yes, Oscar fish can live alone. However, you need to remember that Oscars prefer the company of other fish to solitude. The good part is that you can circumvent that preference by providing Oscars with optimal living conditions.

Keeping a Single Oscar Fish

So long as the Oscar is happy and healthy, it won’t necessarily feel the need to share its living space with other creatures. Here are some of the environmental conditions that you need to pay attention to:

– Tank Size

Oscars are big fish that grow fast. A typical Oscar can reach 16-20 inches in length and grow up to 1 inch per month. You can keep one Oscar in a 20 or 30-gallon tank, just keep in mind that you will have to upgrade it soon.

Ideally, an adult Oscar fish should have at least 55 gallons of water volume available. This is enough to accommodate this not-so-gentle giant in the long run.

– Ideal Water Conditions

The water temperature should revolve around 75 to 80 F with a pH of 6 to 8. Just remember that Oscars are tropical fish and don’t do well with low temperatures.

They share similar living conditions to guppies with slight differences here and there. Just make sure you don’t take this specification as a suggestion to pair Oscars with guppies.

The size differences between the two will spell disaster fast, with guppies being on the receiving end.

– The Diet

Oscars qualify as omnivores but barely. They steer more towards a carnivorous diet with occasional plant matter here and there. I recommend feeding your Oscars a varied diet consisting of:

  • Live food – Live food packs the most nutrients, but it can also carry dangerous pathogens which can hurt your fish. Only get live food products from reliable sources or rely on home-grown live cultures for a plus of safety.
  • Frozen food – Frozen food products are convenient and pack pretty much the same nutrients as live food. With the upside of coming pathogen-free.
  • Plant-based foods – Oscars prefer a carnivorous diet but will also have some fruits and vegetables once in a while. Only feed them fruits and vegetables like bananas, cucumber, spinach, or apples occasionally, as treats.

– Entertainment

This may sound weird, but Oscars are very inquisitive fish who like to remain active and constantly seek entertainment. Pretty much similar to a dog. They’re also known as water dogs for that reason.

You can use a variety of tools to keep your Oscar busy and entertained throughout the day. These moss balls, ping-pong balls floating at the water’s surface, various sinkable toys, etc. You can even have a mirror placed inside or near the tank for Oscar to see its reflection.

The Oscar can’t recognize itself in the mirror, but it will keep it occupied by making it think there’s another fish in the tank with them.

Do all these things, and your Oscar fish will live alone, healthy and happy over the years. Besides, you can always introduce another Oscar into the equation if your fish gets lonely along the way. Just make sure you provide both fish with enough space, which will imply upgrading your 55-gallon tank to something more spacious.

Will Oscar Fish Eat Koi?

Yes, especially when the Koi is younger and smaller. Adult Kois are typically double the size of an Oscar, so it’s unlikely that an Oscar will eat the adult Koi.

Oscars will generally hunt and eat fish that they can consume whole. Aka small enough to fit their mouths.

That doesn’t mean that the adult Koi is safe from the Oscar’s wrath. Like I’ve already mentioned, Oscars are very territorial and often aggressive fish. This can lead them to bully the Koi, potentially causing physical injuries and stressing out the fish.

They won’t kill the Koi, but they will make its life miserable.


Oscars and Kois are not compatible tank mates for all the reasons I’ve already specified, however, there are other fish species compatible with oscar fish, and can be kept in the same tank.

I suggest choosing one and sticking with that one in the long run. The Oscar fish has a lifespan of around 20 years, while the Koi can outlast you. Your typical Koi may live up to 50 years or more under the right conditions.

Whichever fish you will choose, provide it with care, love, and optimal environmental conditions, and it will repay you with attachment and recognition.

avatar Noah
I’m Noah, chief editor at VIVO Pets and the proud owner of a playful, energetic husky (Max). I’ve been a volunteer at Rex Animal Rescue for over 2 years. I love learning and writing about different animals that can be kept as pets. read more...

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