Culling is a process that involves separating desireable fish from nondesireable fish and getting rid of the nondesireable ones by euthanizing them. Generally anything that is wrong with a fish such having a bad eye, deformed fins, being born with the wrong number of fins, growing at a rate conisderable faster or slower than the rest of the brood, and other scenarios would result in that fish being culled. Cullling doesn't always refer to killing unhealthy fish. Sometimes a breeder is looking to exploit a certain phenotype such as a different eye color for example. The breeder has no use for the other fish which lack this eye color and if he cannot find suitable homes for the healthy fish, they are culled as well. It takes resources to keep fish around and keeping unhealthy fish isn't worth it in the long run as it costs money, takes time, and weakens the gene pool as a whole. It is better to produce a dozen healthy and vibrant offspring from a brood of 300 than it is to have 250 inferior fish out of that same brood.