Fish shouldnt just die on you like that, make sure there is plenty of oxygen and a cycled tank. You can't just add a fish to a tank right away because the shock of new water will most likely kill them, you have to add water to the bucket the fish are in so they get used to the water, i add a cup of water every 10minutes for an hour until i feel the fish is ready, i've never had one die in transit. I kept a black crappie and 3 sunfish alive in a 5gallon bucket for 3 days while moving once they were perfectly fine, i travelled with a air pump and constantly added and removed water.