There shouldn't be a need to kill baby guppies! SOMEONE has fish that will eat them, at least!
FWIW, I used to put larger fish to euthanize in a fish bag, put that in a dark paper bag, put them in the 'fridge for 30-45 minutes to slow down metabolism, them move to freezer. Now I put them in a net and slam it onto a solid surface. Anything smaller, I feed off. I have an alligator snapping turtle, so ridding all but the largest fishes is no longer an issue.
Nothing good happens fast in an ecosystem.