keeping fish alive

Question
I am trying to keep three lousy fish alive for my son.  This is our sencond attempt and once again, they are belly up within one week.  We have a simple one gallon tank with air filter.

They were platy's. Nothing special but it sure is tough for my son to see them go.  

Answer
Ruth,

when you got the fish in a baggy and you got home, did you put the bag of fish on top of your tank water for about 15 minutes so that they can get use to the water and the new water temperature?

if not, then that it why. When 15 minutes id up, you can safely place the fish in the tank.

thanks.
bethe.