For our more scientific readers, a study on long fins and how they were passed from one generation to another. Included is just a bit of history of the developement of the butterfly koi.  
 
"The Japanese ornamental koi carp Cyprinus carpio is one of the most popular decorative fish in many countries all over the world, including the United States. Koi were developed approximately two centuries ago in Japan (Kuroki 1981; Davies 1989). The long-fin variety of koi, also called butterfly koi, is a relatively new morph that has been developed in the last several decades.
 
https://www.researchgate.net/publication/232876925_Inheritance_of_Long_F...