Culling is the process of removing animals from a group based on specific criteria. This is done in order to reinforce certain desirable characteristics and/or to remove certain undesirable characteristics from the group.
A "cull" refers to a specimen that has been removed due to an imperfection.
Before breeding clownfish, I did not know how to choose a perfect specimen. I thought that if the fish was 'pretty', healthy and eating, all was well. After raising a few hundred, I begin learning about good and bad characteristics and that culling was a necessary evil that I had to perform.
The responsibilities of an ethical fish Bbreeder are many. The first of which is to produce healthy offspring that represent the species well, are hardy and long lived. Just like breeding dogs, horses (I have done both), etc., a breeder should strive for well conformed offspring that display the desired qualities. When breeding fish, there are many factors that may affect proper conformation such as diet, water quality, toxins and genetics. Whatever the reason, any offspring that display poor qualities such as (just to name a few) flared gills, malformed body or head, under/over bites, and in some instances color patterns, should be euthanized. The additional white on "Designer clowns" are an exception as they are rare and in high demand. Selling sub-par fish to the public will undermine the captive breeding effort by spreading the thought that captive bred fish are of poor quality. This is not the route that breeders want to take. We, as breeders, have the responsibility to provide top quality fish and not just any fish that manages to grow to a saleable size.
Cull The Herd!
Just do it. Nature provides a means to cull any defective fish so breeders need to step up and do so also. When have you seen a wild caught fish with deformaties? I have never.
It is not easy to euthanize them and I struggle with it still (and always will) but I feel it is my responsibility.
There are rare stituations where I choose not to euthanize if the fish only hase a slight deformity (that most people would not even recognize). For example, a slight fin deformity. These 'misfits' are labeled 'culls' and are sold at a highly discounted price or given away.