Koi Information

Tosai Koi

are young Japanese koi in their first year of life, typically under 12 months old, spanning from birth until the following spring (roughly up to 1 year of age). They are often 6–12 months old when sold, providing a high-value option for keepers to watch them grow. 

Key Details of Tosai:

  • Age Definition: Defined as 0–1 year old, or specifically the first winter/spring of their life.
  • Growth Phase: During this stage, they are in a strong first-year growth phase, often reaching 6–8 inches.
  • Appearance: Because they are young, their colours (beni/red) and patterns (sumi/black) are not fully developed.
  • Significance: Popular for beginners and enthusiasts because they are affordable and offer the excitement of observing development.
  • Terminology: After their first year (the following spring), they transition to being called Nisai (two-year-old). 

"Tosai" literally means "this year's, age" in Japanese, representing a fish that is still in its first calendar cycle of life. 

 

Nisai Koi

is a Japanese term in koi keeping that refers to a koi fish in its second year of life (typically 18 to 24 months old). They are considered young adults, having moved past the "Tosai" (first-year) stage, and are highly valued by hobbyists for their developed body structure, emerging patterns, and potential for further growth. 

Key Details of Nisai:

  • Age Definitoin: Generally, 1.5 to 2 years old, often harvested from mud ponds around October or November of their second year.
  • Development: By this stage, the koi's sumi (black), beni (red), and shiroji (white) patterns become more defined compared to their immature Tosai stage.
  • Physical Traits: Breeders look for a strong, robust frame, with a broad head and shoulders.
  • Variety: Many varieties are sold at this stage, including Showa, Kohaku, Goshiki, and Doitsu.
  • Value: Nisai are often preferred because they are established enough to show their potential for future shows (Tategoi), yet young enough to grow significantly in a new pond. 

Nisai follows Tosai (under 1 year) and precedes Sansai (third-year) and Yonsai (fourth-year). 

 

Sansai Koi

are Japanese koi in their third year of life. Generally aged between 2.5 to 3 years old, they are considered mature, having well-developed bodies and stabilized colours, often preferred for their established, high-quality patterns and significant size development. 

Key Details of Sansai Koi:

  • Age Definition: Born in the third year prior to the current season (e.g., in 2026, a Sansai is usually born in Summer 2023 or Spring 2024, depending on the breeder's terminology).
  • Development: They have matured beyond the rapid growth of Tosai (1st year) and Nisai (2nd year).
  • Colour and Body:

 Their colours are generally set and less likely to change significantly, unlike younger koi. Their body shape is typically more refined.

  • Pond suitability: They are excellent for established, large, well-filtered ponds.
  • Terminology Sequence: Tosai (1st year) -> Nisai (2nd year) -> Sansai (3rd year) -> Yonsai (4th year) -> Gosai (5th year). 

Sansai are often considered prime specimens because they offer a blend of mature beauty and continued growth potential.