I also breed and keep goldfish, which are not "natural" either, but nobody seems to have a problem with them. Probably because these fish were created hundreds of years ago, and they've become accepted. I also keep a betta, which has also been manipulated to create the long flowing fins and bright colors. These fish are also acceptable to the general aquatic hobby.
Dogs and cats and birds are breed the same way, yet most people have no problem with this. So if you like the fish get it. Whether you buy it or not, some animal somewhere is going to be selectivley bred to make certain attributes more appealing to the human eye.
There are huge differences between "Selective" or Line Breeding as it is called for selected traits and "Hybridization".
One is the mixing of species. The other is the selected breeding of certain individuals within a species to obtain/maintain a desirable trait.
The unscrupulous practice of either can cause great harm to the animals/fish being bred.
Through repeated in-breeding to maintain a line or develop a trait, be it long fins, no dorsal, sloped hindquarters or black spots, often many recessive genes in a "Pureblood" arise, physical deformities and/or disposition to certain diseases are results.
Goldfish (A very generalized term) and Koi have been bred for literally thousands of years, and yes, there is/has some/been hybridization, and there most definitely is highly selective breeding.
Much harm has been done to some of the more fancy varieties, much intentional breeding of deformities purely because some thought it looked good or got attention.... $$$$
Bettas, there are two species, Betta splendens and Betta imbellis, I don't know whether or not any,or how much, hybridization occurred between these two initially. But most strains are/have been, selectively bred. Show quality Bettas are painstakingly bred and have bloodlines even, while the commercially mass bred Veiltails are so overbred, inbred and mis-bred that physical harm has been and is being caused to these fish.
I will give credit where credit is due, many types of Koi and Golfish are extremely valuable and have bloodlines as well documented and pure as any Kentucky Thoroughbred Racehorse.
Proper and carefully conducted selective breeding over time can and has resulted in new "True Breeds" of Fish, dogs, cats, horses and domestic livestock of all types.
But enough! I'll get to the point...
Selective breeding and Hybridization are not the same.
There is Good and Bad selective breeding.
There is Intentional and Unintentional hybrid breeding.
Selective breeding can hurt a species or strain of that species.
Hybrid breeding can hurt the offspring and cause a species to disappear, forever..... Forever!
Unfortunately, any distribution of hybrid fry/fish is dangerous due to the lack of control and we are already slowly losing certain species.