This is a very good short article on the status of koi in Japan, an introduction to the culture regarding koi. 
 
"Koi have been loved in Japan and they have been characterized by strength and success for a very long time. Koi fish stands for basically anything that is good; luck, prosperity, vitality, and longevity and is very closely associated with the Japanese cultural identity." 
 
https://blog.japanwondertravel.com/koi-fish-japan-29657