How to Humanely Euthanize Your Oscar Fish Using Clove Oil Buy clove oil on eBay PLEASE NOTE Freezing fish, or putting them in very cold water is often thought to be a humane way of despatching fish. This is not the case, this is purely done to preserve the quality of the flesh for eating. Killing fish by flushing them down the lavatory, boiling, or just leaving them out all the water until they die is very inhumane. A person may be committing an offence if they cause the animal pain and distress prior to, or during the killing procedure.