For keeping koi, various water sources can be used, including surface water (such as rivers and lakes), groundwater (such as well water and spring water), tap water, and rainwater. To ensure the vibrant colors and luster of koi, it is essential to adjust the water quality to an ideal state.

Different water qualities affect koi differently. Key factors influencing water quality include pH, carbon dioxide levels, and other parameters.

Water treatment reagents primarily include water conditioners, water purifiers, active nitrifying bacteria, and water clarifiers. There is a wide variety of water testing kits, mainly used to detect concentrations of substances such as chlorine (Cl₂), carbon dioxide (CO₂), copper ions (Cu²⁺), ammonia (NH₃), ammonium (NH₄⁺), nitrite (NO₂⁻), nitrate (NO₃⁻), and pH.

Excretions, uneaten feed, and external debris in the water can decompose and ferment due to microbial activity, leading to deteriorating water quality. Therefore, regular water changes are necessary. Additionally, koi urine primarily consists of ammonia, which is harmful to koi in water. Water changes are also one of the measures to reduce ammonia levels.

When performing a complete water change, disconnect all equipment from the power source. Transfer the koi to a container with new water at a temperature similar to the original. Add an air pump with an airstone to oxygenate the water and prevent hypoxia during the change. Next, drain the old water from the tank using a siphon, and remove and clean/disinfect any gravel and aquatic plants. Clean the glass walls of the tank thoroughly, then fill it with new water (new water refers to tap water that has been aerated for 2–3 days or treated with baking soda—approximately the size of a grain of rice per 50 kg of tap water—to remove chlorine and left to stand for at least half a day). Soak the koi in the container in a 1%–2% saltwater solution for 5–10 minutes. Finally, replace the aquatic plants and gravel, and return the koi to the tank.

Typically performed every 1–3 days, this involves using a siphon to remove fish waste, uneaten feed, and old water from the tank. The amount removed should be about 1/10 to 1/4 of the total water volume. If water quality is poor, 1/3 to 1/2 of the water may be removed. Then, slowly add new water of the same temperature and free of chlorine. Alternatively, a filter can be installed in the tank and run regularly for 1–2 hours, once or twice a day. This helps maintain good water quality and reduces the frequency of both partial and complete water changes.