And please AquaAndrew use some paragraphs or spaces between thoughts or sentences so I don't go blind. It might be easier to write it but had an awful time trying to follow your block wall of text, to read it!!!
Almost everyone's comments were good, thought provoking to referring back to the topic /OP's issue.
I am not a medium or large scale breeder. Just a few little creatures for hobby and enjoyment. When there was a deformity in my guppies, I put them in a different tank, only one lived past youth and she went into a tank with no other guppies and lived for quite awhile.
Killing creatures for no reason is awful. I am not arguing that illness or deformity (not including color as a deformity) or other whatever thing is wrong, is bad to do.
Putting down a pet is gut wrenching, even fish. I had to put down my 17 year old dog, but didn't want to but he was suffering... I still think it was wrong morally, but best for him. So you could say I am conflicted. It was discussed by the vet and she explained it was a kind and loving thing to do for him.
Be it horses or fish or dogs, puppies or kitties, (or people) life is precious.
Back to fish...perhaps there are some other avenues to explore, like fish club members willing to adopt certain types and fish stores who would take some too.
I have adopted fish from people an a number of occasions usually because people didn't want them, they were moving far away, their fish tank didn't sell (thought the fish would GO with the tank), man couldn't sell the tank with the fish in it 125G FW
tropical and everyone wanted it for Cichlids and didn't want the fish (guy was retiring after 10 years of having the tank), and so on. So lots of reasons, some better than others.
Sometimes it was a great decision and other times bad.
OP has a business and should try to focus on ways to funnel the flow away from people who would not treat them properly as a kindness to fish, but it is not the OP's RESPONSIBILITY to do so, it can however be a choice to do so.
One of the people I worked with said oh we had a goldfish and my son got tired of it and we flushed it... People like that should be educated that fish are not throw aways. Petco will frequently take fish in based on their available space, called adoption fish. (She didn't know/think it could be wanted by someone else)
This sort of added service, whether it is simply information, a pamphlet you might prepare, or a person (like from a fish club), web site you can refer them to, makes you an even better business person/resource, AND a more responsible and caring individual as well. (Whether or not you take the fish yourself) You can turn your great service into a future business marketing opportunity, to their friends/associates. Because you took the time to educate, inform, help them.
Just a thought!
No on just euthanizing healthy fish.