While fish mobs don't have idle sounds, at the same rate as regular ambient sounds, messages are printed into console such as:
Unable to play empty soundEvent: minecraft:entity.puffer_fish.ambient
Unable to play empty soundEvent: minecraft:entity.tropical_fish.ambient
Unable to play empty soundEvent: minecraft:entity.salmon.ambient
and
Unable to play empty soundEvent: minecraft:entity.cod.ambient
Intended, sounds will eventually be added.