someone plz help me,i have a fish,what should i do?

ok,here is the story:
my dad bought some dead fishes,so my mom would cook them & then we would eat them...but something went wrong....2 of those fishes were alive (i don't know how they survived without water),& my mom couldn't kill them,so she let them alive in a box with water

this happened yesterday,but today morning 1 of the рыба died,so there is 1 alive....plz help me,what is he supposed to eat?should i clean the water everyday?,& what should i do?

until he dies,or until we send him/her somewhere