At the initial part of the book "Of Mice And Men" Slim described Lennie as a man who still behaves like a child and cannot cause any sort of harm to people. After the fight that took place between Lennie and Curley, Slim bluntly refuse to relieve Lennie of his duties because he valued and appreciated the amount of work done by Lennie in the ranch. Slim also gave Lennie one of his puppies because he knew that Lennie loved soft and small animals.
Because the fish was dying it represented the grenade without its pin.Therefore you have little time to throw it before it explodes and in this casethe fish was suffocating and had a set time before it died.
