What happens next?

How to humanely kill a fish
Ask a veterinarian if possible.
There are many species of aquarium fish, and for most of them the ideal drug dosages and other conditions are unknown. A veterinarian is trained to notice signs of suffering and adjust the process if necessary, giving your fish a less painful death.
Only methods reasonable to perform at home are included in this article. Other methods, such as electrocution and lethal injection, are difficult to perform humanely without training, and/or can harm the person performing them.