Most places require a sample of water for a refund on fish or exchange if the fish purchased from there have passed away. It's pretty easy, you don't need to make one. You just fill a cup with water or a container and put a lid on it. Make sure you have a full cup though because they'll need approx. 5mls for each test for ammonia, nitrite, nitrate, ph, and the 2 hardness tests. I'd say the fish just died from 'new tank syndrome', where they die from no apparent reason when the tank is new. I wouldn't say it is ammonia levels because the fish have only been there for a day, and you don't normally see ammonia readings until about a week after adding in fish. Unless you have added in too many fish. You shouldn't add in more then one or two fish at a time per a week. It's also possible you have added in too many fish for the tank size you have. What kind of fish do you have, how many fish, and what size tank?
If the tank is new, the likely hood of them allowing you to exchange the fish or getting a refund is very low.
