Koi fish generally require 6 - 10 hours of light exposure per day. Specifically, koi have a relatively low demand for light. Six to eight hours of light per day is sufficient, which can prevent excessive growth of algae in the water and stress in koi. However, to enhance the coloration and health of koi, you can place the fish tank in a well - lit area for light exposure, ensuring each light - exposure session lasts about 5 hours.
In addition, koi fish prefer bright environments because bright light can help them hunt for food more quickly, promote their growth and development, and make their body colors more vivid. However, overly long light exposure may lead to an increase in water temperature and changes in water quality. Therefore, it is necessary to properly control the light - exposure time. In summer, long - term direct sunlight should be avoided. Koi can be exposed to light for a certain period in the morning and afternoon, but the intense sunlight at noon should be avoided.