I've got no problems euthanizing a fish. I've had to euthanize higher order critters as well. I live in West Texas. Out here it's cattle country. Lots of folks raise herds of beef cows. We also have folks that raise sheep and goats. Goats are the worst. Those animals forget they have horns and stick their heads through fences, gettnig themselves stuck. If they fight and thrasj around after that, it can get bad. Broken limbs, neck and backs; you name it, they do it. So, in hopeless cases as a general rule, you put them down as quickly and humanely as possible. This goes for your livestock and your neighbors. Most folks understand putting an animal down is just part of it.
I know people don't have gun racks in trucks anymore, but so many people never realized they were there for a reason. Nothing sucks worse than having no gun and needing to put an animal down. I always carry a knife with me though and I've had to dispatch a couple of animals that way. I don't like it being so up close and personal.
The absolute worst was one animal I didn't even put down myself, but rather the vet did. It was my cat, Samantha. I got her when I was 14 years-old. She lived 16 years. She was as much a part of my family as my brother and in fact, lived with me longer than he did when you factor in our age differences and the time he moved away from home. She just got too old. Her kidneys failed. And so, I took her to the vet, they ran what tests they could and determined there was no way to save her. I left work at lunch and went to the vet's office to be with her when she went. My mom came too. We cried a long time. Sammy just went to sleep, but at least I was there pettnig her and she knew I loved her and she would always be my kitty.
I know how hard it can be to euthanize an animal, especially a pet. I feel for you, Bill.