If the water quality of koi fish is poor, the best treatment method is to change water properly. No matter what fish you raise or what fish tank you use, the water quality will deteriorate over time, so you need to change the water regularly. Usually, it should be changed every other week. If the fish tank is small, the water change period can be shortened, and it can be changed once every three to five days. You can't change too much water every time, but you can change about one third at most. If you change too much, it may stimulate the fish.

1. Natural deterioration: The water quality of fish tank is inevitable, because compared with natural water body, its volume is too small and its feeding density is relatively large, so the water quality will deteriorate naturally, which is difficult to avoid. Through filtration and other means, it only alleviates the speed of water quality deterioration.

2. Too many filter materials: Fish tanks generally have filtration. Although the more filter materials, the better the filtration effect, after the number of filter materials exceeds a certain limit, nitrifying bacteria will multiply in large numbers, resulting in excessive nitrate, which will cause acid drop in water bodies and affect water quality.

3. Algae outbreak: If there are too many algae in the fish tank, they will die out quickly when they don't get enough nutrients, which will also affect the deterioration of water quality at this time. Therefore, after seeing algae, it is usually necessary to remove it at the first time to avoid excessive reproduction.