I read the following in a water gardening magazine about butterfly koi: “Produced by crossing comet goldfish with koi carp, a butterfly koi proves to be hardier and smaller than its larger koi cousin.” Do you agree? Everyone I have talked to about this claims that they grow just as large as other koi.
As a generalization it is probably correct. Comets, which are goldfish, are hardier than koi. They handle colder water temperatures better and they tend to handle lower dissolved oxygen levels better too. They are also definitely smaller than full-grown koi.
If you cross a comet and a koi, the offspring will tend to mix the genetic traits of both. On average, they should be larger than comets, but smaller than koi. They should have a hardiness somewhere between the two.
They should also have more coloration than comets. Many will have the flowing fins of fancy comets — but others will not.
Remember that breeders select for certain traits. They pair fish to maximize color, fin structure, or whatever they find interesting. (Others just let nature decide what happens and then they simply cull what will not sell. But this is still genetic mani<$>pulation.) Therefore, the characteristics of the butterfly koi available for purchase can vary widely from dealer to dealer. There are no standards.
There is also a down side to this genetic manipulation. A breeder pushing for a specific color pattern or finnage structure may also inadvertently introduce other genetic codes systematically into his or her fish. These animals may be less tolerant of cold than either comets or koi. They may be prone to physical deformities, tumors or be sterile.
I am not surprised by the observations of your friends. If butterfly koi seem to reach the same size as koi, it is probably because most hobbyists never get to see full-size koi. Too many pondkeepers think the natural life span of these animals is five years (because that is how long they last), and these tend to be 12 inches in length. By contrast, a nice 20 -year-old koi, which is still a teenager in koi years, is big — more than 2 feet for just the body.