Hello, Fishtastic. Welcome to the forum! 
When fish died within hours, it is most likely due to either chlorine or shock.
Did you use the aquarium water conditioner according to the instruction on the bottle? What water conditioner are you using? While it is unlikely you have overdosed, but I just want to make sure this is not the problem.
Did you acclimate your fish when you add them to the new fish tank? Fish acclimation is critical when you introduce new fish to a new tank.
The water fish came with has different water temperature, PH, hardness, among other things. If the difference between the water in the plastic bag the fish came in with is too big compared to the water in the fish tank, the fish will get into shock and may die in hours.
For details on fish acclimation, you may read the following article -
3 Important Steps to Introduce New Fish to Your Home Aquarium
There is also another article on the possible causes of fish death. Why did My Fish Die so Fast?
You might want to give it a read if you haven't.
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