The causes and solutions of koi fish's closed fin swimming are as follows:

Reason: It may be that the water was changed, the temperature changed greatly, and I caught a cold.

Solution: Like goldfish, koi fish has no strict requirements on water temperature and quality. The suitable temperature range is 2℃ ~ 30℃, but it can't resist the sudden change of water temperature and the discomfort in koi fish when the temperature difference suddenly exceeds 2℃ ~ 3℃. The optimum water temperature is 20℃ ~ 25℃. It needs sufficient oxygen and is suitable for living in a slightly alkaline and low hardness water environment.