If fish that are to be euthanased are for immediate consumption (e.g. from a fishing trip), they can either use Aqui-S anaesthetic (active ingredient iso-eugenol is a derivative of clove oil which is registered for you in several fish species), or use percussive stunning (some examples of implements sold in fishing shops shown below) followed by bleeding out and/or ikijime (pithing the brain).
Leaving fish in buckets to die slowly or putting them into an ice slurry is considered inhumane.
