I'm sorry that you need to euthanize a fish. I'm not sure about the Eugenol, but the method I use to euthanize a fish is place the fish in a bucket with not too much water, then add several alka seltzer tablets - saturates the water with carbon dioxide - the fish just goes to sleep essentially. The fish do not flinch or struggle, so (I hope) it is relatively humane and painless. And we do NOT need another discussion about "do fish feel pain"? That's not the point. The point here is that when we take on the care of an animal, we take the responsibility to give it the best care possible, and to be as humane as possible regardiing all aspects of its care, including euthanization, when necessary. It's called respect for life. Good luck DD!