would you interbreed your children! same difference, genetics get passed along untill you have a fish or child that is very ill and in fish case strained for a specific difference/charateristic. Its not pleasing to the eye, and most people wont buy them in my view... take the pesky hornflower ( which was umpteen varietys of fish) not pleasingto the eye! just made for money!!
Do you mean two different species like breeding a firemouth with a texan. I had this happen by accident and it wasn't pretty.
But mainly its to keep the stains pure.
