No matter what animals you are breeding you should have a plan and technique for culling animals. I used to raise Canaries and there are government regulations and guidelines on culling animals. Just about the most universal way is to gas them with CO2. I've tried it on snakes before, it generally just knocks them out but after a while they can be revived. It's probably best to gas with CO2 followed by decapitation.

It's the same with euthanizing rodents before feeding, if you freeze them to death or smash them on a hard surface to kill them it's considered inhumane and the long arm of the law can step in and confiscate all of your animals. Breeders should be prepared to euthanize and know which way is morally and legally acceptable. If you breed anything in large numbers you will always get deformities, I bred Angel fish too and had a lot of weird freaky fish hatch out over the years. The amount of deformity before culling is totally up to the breeder.