This could be due to a number of factors – poor water quality, lack of oxygen, parasites and poor nutrition. A food with a high level of stabilised Vitamin C will help build up the immune system and perk up your koi. Also, look for Vitamin E as this helps stress related problems. Soaking the pellets in orange juice will also help. Incorrect nutrition will result in unhealthy koi, regardless of how good the water quality is.
A complete step by step guide to aquascaping for the complete beginner. From design inspiration & styles through to equipment & inhabitants.