The primary reasons to euthanize a fish are injury and disease. A fish can be injured by an aggressive tank mate or a sharp tank decoration that needs removal. A lot of disease can affect aquarium fish, from the common fin rot to dropsy, a disease that causes the fish to bloat. Fish diseases are usually obvious because they significantly alter the fish’s appearance or result in the fish no longer eating or swimming correctly. It is possible to humanely euthanize a fish so that it suffers very little or not at all.
Some aquarium fish are highly aggressive and must be kept alone, or else they will attack their tank mates until none are left alive. Even normally docile fish can attack one another until death of an opponent. It is common for semi-aggressive fish to occasionally injure another fish. These injuries often heal with no intervention needed, though it is usually recommended to separate the injured fish from the aggressors. Not all fish fights have happy endings, though, and it may be necessary to euthanize a fish that has been severely injured and will only suffer until its inevitable death.
A few viral diseases often affect domestic fish and can usually be spotted fairly easily. For example, fish pox can usually be spotted by the appearance of gray or pink growths on the body of a fish. Furthermore, lymphocystis, which is often brought on by poor living conditions, can often be spotted by the appearance of brown wartlike growths. Another condition, called Hexamita, is caused by parasites and is typified by fish becoming lethargic and lesions appearing on their heads. When a fish experiences this condition, it will usually become visually smaller due to poor health.
Clove oil is often regarded as the most humane and simplest way to euthanize a fish. This oil is a substance that can be added to a fish’s aquarium and, if dosed properly, the fish becomes unconscious. It should be kept in mind that utilizing clove oil alone for this purpose is difficult because a large amount of it would be needed. Instead, an alcoholic substance such as vodka can be added to the aquarium after the fish is put to sleep with clove oil. Whatever method is used, it is important to make sure that the fish is actually dead and not simply asleep and to anesthetize and then euthanize the fish in the appropriate order.