Age terms: tosai, nisai, sansai and yonsai
Key facts: Tosai (当歳), nisai (二歳), sansai (三歳) and yonsai (四歳) are Japanese terms describing a koi's age in years, counted from the year it hatched rather than from an exact birthday[1][2].
Koi are not aged the way humans are. According to the All Japan Nishikigoi Promotion Association, a koi is called “tosaigyo” from hatching until around one year old, after which it becomes “nisaigyo”[2]. The American Koi Judges Association glossary gives similar definitions: tosai means “this year’s fish” (born this year), nisai means “born last year,” and further terms continue upward, with yonsai meaning four years old and gosai meaning five years old[1].These terms are widely used by breeders, dealers, and koi shows to describe a fish's generation rather than its precise age in months, since koi hatched in the same breeding season are grouped together regardless of the exact spawning date[1][2].What tosai, nisai, sansai and yonsai mean
The core terms follow a simple progression. Tosai refers to a koi in its first year, born that year[1]. Nisai refers to a koi born the previous year, effectively its second year[1][2]. The American Koi Judges Association glossary also lists gosai for five-year-old koi, continuing the same counting pattern used for sansai (third year) and yonsai (fourth year)[1].The All Japan Nishikigoi Promotion Association notes that this method of counting age differs from how human age is counted, since koi are grouped by hatching year rather than by exact date of birth[2].Why age terms matter when buying koi
According to the All Japan Nishikigoi Promotion Association, nisaigyo (second-year koi) are recommended for first-time koi owners[2]. The koi dealer Kodama Koi Farm states that age affects price, since adult koi between two and four years old are larger and longer and therefore tend to fetch higher prices, while tosai koi, at just one year old, are typically smaller and less expensive[4]. The same source notes that buying young tosai involves some uncertainty, since their adult color and pattern are not yet fully visible, whereas older koi already display the pattern and color a buyer is purchasing[4].Kodama Koi Farm also uses the term tategoi to describe koi of any age judged to have strong future potential, distinct from the simple age-based terms[4]. The AKJA glossary similarly defines tategoi as a koi a breeder keeps because it is expected to improve and increase in value at a later age, noting that breeders do not easily part with such fish[1].Typical size by age stage
Kodama Koi Farm publishes an approximate growth timeline linking age terms to size, though it notes that growth depends heavily on pond space, filtration, feeding, and genetics rather than age alone[3]. Per this dealer's guide, tosai (year one, roughly 6–12 months) typically measure about 6–12 inches, nisai (year two) about 12–20 inches, and sansai (year three) about 20–26 inches, with growth slowing by around three to six years as koi reach adult size[3].| Stage | Age range | Average size |
|---|---|---|
| Tosai | 6–12 months | 6–12 in |
| Nisai | 1–2 years | 12–20 in |
| Sansai | 2–3 years | 20–26 in |
| Adult koi | 3–6 years | 24–32 in |
| Mature koi | 6+ years | 30+ in (if optimal) |
Age terms at auctions and shows
Age terms also appear in commercial and show contexts. A report on the 12th All Asia Cup Koi Show, published by the Dutch site Besems.eu, describes an auction of high-quality nisai Kohaku offered by a Chinese breeder, with fish measuring between 67 and 70 centimeters at that age, described as “promising” quality for nisai[6]. This illustrates that nisai koi can already reach substantial size under intensive growing conditions, well beyond typical hobbyist pond growth[6].The All Japan Nishikigoi Promotion Association also notes that record-setting koi can be identified by age at sale; it cites a Kohaku that was nine years old and 101 cm long, bred by Sakai Koi Farm in Hiroshima, which reached the highest auction price in the association's account[2].Age and health considerations
Age can be relevant to koi health as well as size and price. Kodama Koi Farm states that young tosai (one-year-old koi) are the age group most likely to encounter carp edema virus (CEV, also called koi sleeping disease), and that experienced Japanese breeders often allow healthy young koi controlled exposure so immunity develops before the fish reach the marketplace[7]. According to this source, this practice is one reason many hobbyists never experience CEV even after introducing new koi, though protection is not always complete and the disease can still appear later[7]. Readers concerned about symptoms in their own koi should consult the dedicated page on carp edema virus (koi sleeping disease) and seek a fish veterinarian for diagnosis.Age terms versus lifespan
Age terms such as tosai through yonsai describe only the early years of a koi's life, while koi can live much longer. The All Japan Nishikigoi Promotion Association states that the average lifespan of Nishikigoi is around 30 years, and that with proper care some live more than 80 years, though their peak aesthetic appreciation is estimated at around 12 to 15 years as colors fade and bodies bend with age[2]. In a separate column, the same association clarifies that the widely cited claim of the koi “Hanako” living 223 years, based on counting ridges on her scales like tree rings, is now considered incorrect, since young koi form many growth ridges per year; the association instead states the actual lifespan of Nishikigoi is thought to be around 30 to 40 years depending on environment[8]. More detail on this is covered on the dedicated page about Hanako, the famous old koi and on lifespan and aging.Common questions
What does tosai mean in koi terminology?
What is a nisai koi?
How big should a sansai or yonsai koi be?
According to a growth timeline published by the dealer Kodama Koi Farm, sansai (year three) koi average about 20–26 inches and yonsai-stage koi fall within the broader adult range of roughly 24–32 inches by three to six years old, though actual size depends heavily on pond conditions and feeding[3].
Does age affect the price of a koi?
Yes; the dealer Kodama Koi Farm states that adult koi between two and four years old are larger and tend to cost more than young tosai, while tosai are cheaper but carry more uncertainty about adult color and pattern[4].
Are young koi more vulnerable to disease?
The dealer Kodama Koi Farm states that tosai (one-year-old koi) are the age group most likely to encounter carp edema virus, and describes breeder practices intended to let immunity develop before sale; diagnosis of any suspected illness should involve a fish veterinarian, per the dedicated health pages[7].
Related koi topics
References
- ^ Glossary – American Koi Judges Association (AKJA). American Koi Judges Association. akjajudges.com/glossary/. Retrieved November 2026.
- ^ Q&A - All Japan Nishikigoi Promotion Association. All Japan Nishikigoi Promotion Association. jnpa.info/en/qa/. Retrieved November 2026.
- ^ How to Grow Big Koi: Guide to Large Koi Growth with a Healthy Pond. Kodama Koi Farm (koi dealer). kodamakoifarm.com/how-to-grow-big-koi/. Retrieved November 2026.
- ^ How Much Do Koi Fish Cost? Koi Pricing Guide for USA. Kodama Koi Farm (koi dealer). kodamakoifarm.com/how-much-do-koi-fish-cost/. Retrieved November 2026.
- ^ Keep Nishikigoi | Zen Nippon Airinkai. Zen Nippon Airinkai. zna.jp/en/keep-nishikigoi/keep-nishikigoi/. Retrieved November 2026.
- ^ Jureren tijdens de 12e ALL ASIA CUP KOI SHOW - Besems.eu. besems.eu. besems.eu/2019/05/09/jureren-tijdens-12e-all-asia-cup-koi-show/. Retrieved November 2026.
- ^ What is Koi Sleeping Disease? KSS, KSD, Carp Edema Virus (CEV) Symptoms & Treatment. Kodama Koi Farm (koi dealer). kodamakoifarm.com/koi-sleeping-disease/. Retrieved November 2026.
- ^ Ancient koi “Hanako” - All Japan Nishikigoi Promotion Association. All Japan Nishikigoi Promotion Association. jnpa.info/en/column/25/. Retrieved November 2026.