What causes fish to swim in circles?

Yesterday at a koi pond I saw a fish swimming wildly and fast, darting around, occasionally jumping out of the water or spinning in circles in the water. And it did not stop for a long time. Afterwards I used a net to restrain it before it stopped. Because this fish was too expensive I did not dare to touch it. Could all you experts please tell me what causes can make a fish show this kind of condition. The more detailed the better. Thank you.