When you get a new fish tank it have to be cycled this means running the tank whitout any fish in it, this is to make the bacteria that breaks down the waste from the fish and food time to grow in the filter.
It is quite common that pet shops don't inform you the customer of this.
And this do almost always end in dead fish.
this link will help you understand how this works and how to cycle the tank.
To cycle a fish tank fill the tank with water start the pump/filter and heater.
Add a tiny bit of fish food, you now have to get a water test that shows amonia
nitrite and nitrate I do use test strips they are easy to use.
Now you have to test the water every two to thre days, what is expected to happen is you will be able to see the amonia readings on the test rise but not the nitrite,
After several days the readings on the test will show both amonia and nitrite in the water you now add a litle more fish food.
Some days after this the test will show less amonia but higher level of nitrite.
now you simply take a water test every two to thre days until the amonia and nitrite level is close to zero when this happens you will be able to measure some nitrate in the water this is OK and your fish tank is now cycled and ready
for the fish!.
When you have put the fish in the tank it is best to not feed it for a day and when you start feeding it start with very little food.
If you feed more then the fish eats in five minutes the amonia and nitrite might come back if this happens stop the feeding and change half of the water and test the water every day and cange some water if the readings is still high.