There are many interconnected issues with a "new" aquarium and I think previous responders have identified the issues, but in order to determine if one or a combination of these are causing fish to die within hours we will require some more basic information. Taking only
the latest attempt, could you please tell us:
-what specific fish and how many were put in the tank, and when (in relation to initial setup and acquisition of the fish if not all together)?
-are there any live plants?
-how did you cycle the tank?
-what is the pH and hardness of your tap water?
-what is the pH of the tank water?