Euthanising fish humanely is an important welfare skill for aquarists and aquaculture workers — many commonly used methods cause prolonged suffering rather than rapid death.
Fish euthanasia is a welfare skill that many aquarium hobbyists and even some professionals lack — leading to well-intentioned but prolonged suffering in sick and injured fish. The common methods that cause suffering (flushing, air exposure, ice water for tropical fish) persist partly from ignorance and partly from undervaluation of fish experience. Clove oil overdose is inexpensive, readily available and produces quiet anaesthesia before death — a meaningful welfare improvement over any method that prolongs dying. The welfare principle is the same as for any animal: euthanasia should minimise the time between intervention and unconsciousness.