Generally, after a period of rapid growth throughout spring, summer, and autumn, koi tend to display their best condition when the water temperature drops to around 15–20°C in autumn. During this time, the white ground (shiroji), black patterns (sumi), and red patterns (hi) all appear in optimal form—vibrant, crisp, and well-defined. This is considered the time when koi look the most beautiful, which is why most major koi shows are scheduled to take place under these temperature conditions.
The stable, mild temperatures of autumn help intensify pigmentation, enhance skin luster, and bring out the full contrast and depth of colors, while also allowing the fish to showcase their most balanced body condition. Thus, autumn is not only the peak season for koi aesthetics but also the preferred time for competitions and appreciation.
