What Are Koi Fish? Simply Amazing Living Jewels

From their fascinating origins to their friendly personalities, koi aren’t just fish—they’re living art that brings peace, beauty, and connection to your outdoor space.

Are you wondering why seemingly normal people often become fanatical about their koi?

Koi are descended from the common carp, a rather unremarkable-looking fish that is usually black, dark brown, or bronze in color.

Today they come in a rainbow of colors and are often referred to as living jewels or Nishikigoi.

Once you see them swimming in clear water you will understand why.

The beauty and grace of these fish and the many colors and marking patterns create truly unique fish.

No two koi are exactly the same. The endless variety of colors and patterns is what makes folks go koi crazy and become breeders or collectors.

Then too, koi can easily live 30 years and have personalities making them more like real pets than other aquarium fish.

Koi learn to recognize their owners and often will hide when strangers approach.

They can even be trained to eat from your hand with a bit of patience.

What Are Koi Fish And What They Are Not

Many people think koi are simply large goldfish. However, they are not goldfish at all.

One look at one with its barbels will quickly show you they are not the same. In fact, if anything they more closely resemble catfish.

These large fish are not suited to aquariums or bowls like goldfish are. A mature koi would require a minimum of 250 gallons of water.

There are folks that have small indoor ponds and keep a few koi and this is realistic.

To better understand these fish, it is helpful to compare their needs with that of a goldfish and a typical water garden pond.

Koi Fish Ponds Vs Water Gardens

Koi are large fish; some are over 36″ when fully grown. They eat a lot and produce a lot of waste.

Because these are so unique, they require a specially designed pond to meet their needs.

A water garden is generally a shallow pond that has a variety of pond plants in it. This is why it is often called a garden pond.

It may be a still pond or have water flow. It may or may not have any filtration. It may not even have any fish in it at all, but simply be a planted pond.

A koi pond on the other hand requires constant water movement and filtration to deal with the waste produced.

It will need to be larger because koi are active swimmers and also much deeper too.

Say Good Bye To Your Water Garden

The garden pond is suitable for goldfish but totally unsuitable for koi for several reasons.

First, it will cease to be a garden pond if you put them in it. While goldfish may nibble at your plants they are relatively easy on pond plants.

Koi, on the other hand, are ruthless when it comes to pond plants. Whatever they don’t manage to eat or tear up on top they will root up from the bottom.

Koi fish can denude and muddy up a garden pond in record time.

If you have ever looked at a traditional Japanese koi pond you will notice very few plants in the water.

You may see a few water iris leaves sticking up or the occasional spatterdock. Generally, koi don’t like to eat these very much, so they survive.

The Japanese have learned to design ponds that work with the koi’s natural instincts and meet its needs and they don’t expect their fish to change to suit their needs.

There is a valuable lesson there for anyone building a koi pond.

Koi Ponds Need To Be Deep

Because koi are large and strong swimmers, they require deep ponds. A koi pond should be at least four feet deep. There are several reasons for this.

First, koi require enough room to be able to swim not just from side to side but also up and down.

It is critical for koi to be able to swim vertically to develop proper muscle structure.

Secondly, they make quite a tempting target for raccoons and herons because of their larger size.

So it is necessary to keep them in deep water to discourage predators.

Third, koi fish cannot handle the fluctuations in temperature or water quality that goldfish can.

A shallow pond will get too hot for koi and larger ponds are more stable and forgiving of changes in water quality.

What Are Koi Fish — FAQs

Are koi and goldfish the same?

No. Koi are domesticated common carp (Cyprinus carpio) and have barbels at the mouth; goldfish do not. They also have different size and pond needs.

How long do koi live?

With good care, many koi live 25–35 years, and some can exceed 50 years in well-managed ponds.

How big do koi get?

Adult koi commonly reach 24–36 inches, depending on genetics, diet, and water quality.

How many gallons per koi do I need?

Plan a minimum of ~250 gallons per adult koi and stock lightly so your filtration can keep up as fish grow.

How deep should a koi pond be?

Aim for at least 4 feet. Depth supports stable temperatures, predator safety, and healthy exercise for strong swimmers.

Can koi live indoors?

Yes, but only in large, well-filtered indoor ponds. Most aquariums are too small for adult koi.

What do koi eat?

A quality koi pellet as the staple, plus occasional treats like fruits or veggies. Feed amounts your filter can handle.

Do koi need strong filtration and water movement?

Absolutely. Target a full-pond turnover 1–2 times per hour with mechanical and biological filtration.

Can koi be kept with pond plants?

Koi uproot and nibble many plants. Choose tougher species in protected planters, or design plant zones separate from fish.

Will koi recognize their owners and hand-feed?

Yes. Many koi learn to associate people with food and can be trained to eat from your hand over time.

What temperatures do koi prefer? Can they overwinter?

They do best in stable, clean water. In cold regions, deep ponds with proper aeration allow overwintering; avoid sudden swings.

What are the main koi varieties?

Popular groups include Kohaku (red/white), Sanke, Showa, Ogon (metallic), Utsuri, Asagi, and many more “Nishikigoi” color types.

What Are Koi Fish? The Ultimate Fish To Keep

If you enjoy aquariums or have a garden pond, then keeping koi will bring everything to a whole new level for you. But be careful because you may get bitten by the koi bug. More on koi fish care or beginners.

If you enjoy beauty and nature, then you will enjoy watching your koi fish swimming in their own pond and sparkling like jewels. As you are listening to the relaxing sound of running water it will soothe your soul. Keeping koi brings all this and more into your life and that is why people get crazy about their koi fish.