Raising koi is an industry that is growing across the world; a growing international trade bringing income to many in a variety of countries. Here is a study from Java, in southeast Asia, that looks at the results of current techniques used to grow koi in Java. Even down to keeping the snakes and frogs out of the ponds. 
 
 
"Abstract. Koi fish (Cyprinus carpio) are one of the most popular ornamental fish commodities in the world. The purpose of this study was to analyze the growth and monitoring of koi fish on natural hatchery techniques. This research was conducted at the Technical Implementation Unit of Freshwater Aquaculture Development in Umbulan, Pasuruan, East Java Province....."              
 
https://www.researchgate.net/publication/331451204_Growth_monitoring_of_...