I tend a flock of 150-200 free range chickens, and hatch an incubator or two of chicks every spring. There's always one or two deformed or otherwise sick chicks in the hatching. I used to allow them to struggle themselves to death. But that's far more inhumane than putting them out of their misery when imminent death awaits them. I kill by drowning, as the most humane method. Though I understand that wouldn't quite work so well for a fish!
Fish squish pretty easy. Just shut your mind off and remind yourself you're acting in its best interest. Good luck with that. I know it's hard.