Asagi and Shusui: everything you need to know about blue koi carp

In a pond dominated by red and white, a blue koi immediately catches the eye. It appears calmer, almost mineral-like, and yet it carries one of the oldest bloodlines in the history of Nishikigoi. The Asagi koi and its Doitsu cousin, Shusui, are the two pillars of the blue koi family: the former is recognized by its scaled, net-patterned back, while the latter has bare skin crossed by a row of large dark scales. Recognizing them is not difficult; judging them, developing them, and understanding why their blue changes from one season to the next is another matter. That is what we explain here, from the perspective of someone who sees many of them pass through a sales pond.

One of the oldest bloodlines

Asagi is one of the very first koi varieties selected in the Niigata region. It descends directly from the blackish and bluish carp raised for consumption, in which color mutations were identified and then preserved. It is often considered one of the ancestors of many current varieties: by crossing it with Kohaku, breeders obtained Koromo, with red patterns edged in blue, and Goshiki, with five blended colors. With the arrival of European mirror carp in Japan at the beginning of the 20th century, they later produced its Doitsu version, Shusui.

This age has a practical consequence: Asagi retain much of the wild carp. They are generally hardy, grow well, and reach impressive sizes. They are also less “standardized” than Kohaku or Showa, which explains the wide variety of fish encountered, from almost white blue to navy blue.

Where does the blue come from? A little skin biology

No freshwater fish in our ponds produces blue pigment. A koi's color results from specialized cells called chromatophores: melanophores contain black pigment, while xanthophores and erythrophores contain yellow and red pigments (carotenoids supplied by the diet), and iridophores contain small crystals that reflect light. What we see as blue in an Asagi is, according to the most widely accepted explanation, black pigment located deep in the dermis, seen through layers of skin and cells that preferentially diffuse short wavelengths. The same phenomenon gives veins under human skin their bluish tint.

Above all, remember what this implies: an Asagi's blue is black "seen from a distance." If it rises toward the surface, the fish appears darker, almost slate-colored. If it sinks or disperses, it becomes paler. And because melanophores react to stress, light, temperature, and water chemistry, the blue of the same koi can vary significantly from one month to the next.

The Asagi's scale reticulation

The Asagi's iconic pattern is its network. On the back, each scale has a deep blue center and a lighter, almost white edge: together they form a very regular net, like a fisherman's mesh laid over the fish. This net should begin just behind the head and descend uninterrupted to the tail, symmetrically on either side of the dorsal fin.

The red, which the Japanese call hi, is classically found in three places: the cheeks, the lower flanks below the lateral line, and the base of the pectoral fins. The fins themselves may be entirely or partially orange. The contrast between the cool blue of the back and the warm red framing it is what gives the fish its charm.

Asagi subvarieties to know

Names vary slightly depending on breeders and competitions, but the main families are stable. The table below lists those actually encountered in garden centers or from European importers.

The Shusui also has comparable varieties: Hana Shusui, with an additional red band between the dorsal line and the lateral line; Hi Shusui, whose red covers a large part of the back; and Ki Shusui, where the red is replaced by yellow. There are also Shusui with reflective scales, as well as metallic forms classified elsewhere in competitions.

Asagi or Shusui: how to tell them apart with certainty

At first glance, a pale Asagi and a Shusui can look alike, especially in slightly cloudy water. The test is simple: look at the skin between the dorsal fin and the lateral line. If it is covered with a network of tightly packed scales, it is an Asagi. If it is smooth, with only one row of large dark-blue scales on each side of the dorsal fin and possibly another along the lateral line, it is a Shusui.

The two fish are not judged on the same points. For the Asagi, everything depends on the regularity of the net pattern and the cleanliness of the head. For the Shusui, the dorsal scales are examined first: they should be aligned and uniform in size, with no scattered stray scales on the flanks. The blue of the bare skin should be uniform, without darker patches.

Another difference that is often overlooked: the Shusui, like all Doitsu koi, has less-protected skin. It is more sensitive to rubbing against walls, handling with a landing net, and, in a shallow pond without shade, prolonged exposure to sunlight. A Shusui that returns from transport with marked skin will take longer to regain its appearance than an Asagi.

Quality criteria, point by point

Whether you buy a young fish for a few dozen euros or a more developed specimen, the criteria remain the same. We recommend observing the koi from above in a clear tank, then from an angle to inspect the flanks.

1. The head: it should be clear, bluish white or very pale gray, with no dark spots (enthusiasts call them shimi). This is often what separates an ordinary Asagi from a beautiful Asagi.
2. The reticulated pattern: regular and well defined, covering the entire back up to the caudal peduncle, with no blurred areas or isolated darker scales.
3. The hi: crisp, a clear orange, distributed symmetrically across both cheeks and both flanks. Hi that “bleeds” into the blue is less desirable.
4. Pectoral fins: clean, with symmetrical hi at the base if present.
5. Body shape: as with all koi, a powerful, well-balanced body, with no spinal deformity or disproportionate head.

A classic pitfall: the very light young Asagi, with a subtle reticulated pattern, that appears “washed out” in a sales pond. These are sometimes the fish that develop the most beautiful pattern after a few seasons. Conversely, a very young koi that is already dark may turn slate gray. If the purchase is based on the hope of future development, ask the seller which bloodline the fish comes from and how its siblings have developed. To get an idea of the price differences between a pond fish and a show-quality specimen, our article on how to estimate the price of a koi carp provides useful benchmarks.

How a blue koi changes with age

The Asagi is a variety that changes. That is precisely what fascinates enthusiasts and confounds beginners. Three trends are common.

The hi rises. In many bloodlines, the red on the flanks slowly progresses upward along the body over the years. A koi with very low hi at two years old may, by seven or eight, have red reaching the lateral line or even extending beyond it. This is not a disease; it is the fish's genetics expressing themselves.

The blue darkens. With age, the black pigment deepens and the back appears darker. The reticulated pattern remains visible, but the contrast between the center and edge of each scale decreases. Some Asagi retain a light blue throughout their lives; others turn slate gray within a few seasons.

The head can become marked. Small dark spots sometimes appear on the skulls of fish that previously had none. They may be linked to genetics, but also to periods of stress or poor water quality. Some fade when conditions improve, while others remain.

To track these changes without relying on your memory, photograph your koi once or twice a year, always under the same conditions: same tank, same time, same natural light. After three years, you will have a genuine assessment of your fish, far more reliable than any remote opinion.

Water and color: what really matters

You hear everything and its opposite about the influence of water on blue. Here is what we can say with caution, based on biology and observations from very different ponds.

The point that comes up most often in our discussions is stability. Water that is perfectly “ideal” on paper but fluctuates sharply between morning and evening, due to algae photosynthesis and an overly low KH, damages the color more than slightly less perfect but stable water. Alkalinity acts as a buffer here. If your measurements show that KH collapses in summer, our guide on increasing the KH of a pond details the methods.

As for feeding, the Asagi does not need colour-enhancing food at every meal. Carotenoids intensify the hi on the flanks, but they can also give the head and the light edges of the scales a yellowish tint, which obscures precisely what we are looking for. A balanced staple, possibly supplemented in season with alternating colour food, often provides the best compromise. Our article on how to choose good food for your koi covers the preferred composition.

Common mistakes with blue koi

• Assessing the fish in winter or just after transport: the blue is often dull or too dark at that point. Wait a few weeks in water above 15 °C.
• Confusing a “faded” Asagi with a mediocre Asagi: a subtle net pattern on a young fish is not a permanent defect.
• Overdoing colour-enhancing foods: the head, which is supposed to remain light, is the first to suffer.
• Forgetting about the Shusui's skin: a rough surface, an abrasive pond edge and repeated handling leave lasting marks.
• Neglecting KH: pH fluctuations are one of the first enemies of stable colour.

Practical example: putting together a group with blue koi

Common scenario: a pond of fifteen to twenty cubic metres, around ten koi, mainly Kohaku and Sanke, and the desire to add a little variety. One or two Asagi are enough to transform the way the pond looks. Their blue makes the white of the Gosanke stand out and adds a sense of visual calm. If the pond bottom is dark, choose a Narumi or Mizu Asagi, whose light blue remains visible; on a light-coloured bottom, a deeper blue Asagi or a Shusui with a well-defined back will stand out better.

Asagi coexist without difficulty with all other varieties. They are often quite friendly and readily follow Chagoi by hand. Their large adult size simply needs to be anticipated when calculating the pond's stocking level: a five- or six-year-old Asagi weighs as much as a Kohaku of the same age, sometimes more.

Read also

A beautiful Asagi can be worth far more than you might think: How much are the koi carp in my pond worth?.

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Frequently asked questions

— What is the difference between an Asagi and a Shusui?

The Asagi is fully scaled: its back forms a regular blue net pattern. The Shusui is the Doitsu version of the Asagi: its skin is almost scaleless, sky blue, with a single row of large dark scales on either side of the dorsal fin.

— Why is my blue koi becoming darker?

This is a common development with age, and it can also be influenced by water quality, temperature and stress. A blue that darkens uniformly is not a problem; isolated dark spots on the head, however, are an aesthetic flaw.

— Can an Asagi's red spread over its entire back?

Yes, in some bloodlines the hi gradually rises toward the back over the years. If you want a fish with clearly separated blue and red, choose a young one whose hi remains low, below the lateral line.

— Are Asagi more robust than other koi?

They have a reputation for being hardy, long-lived fish, close to the original carp. They remain demanding in one respect: water cleanliness, on which the clarity of their net pattern and head depends.

In summary

• The Asagi is a fully scaled blue koi with a net pattern on its back; the Shusui is its Doitsu version, almost scaleless with a row of dark scales.
• The blue comes from a deep black pigment seen through the skin, which explains its variations with age, water and temperature.
• Quality criteria: clear, clean head, regular net pattern extending to the tail, sharp and symmetrical hi, well-aligned scale rows in the Shusui.
• Over the years, the hi rises and the blue darkens: choosing a young fish means betting on this evolution.
• Clean water, stable KH and a balanced food maintain the blue better than highly coloring food.

Last updated: October 3, 2026.