All new tanks go through what is called a cycle. Fish produce ammonia in their waste which is toxic. They slowly poison themselves with their own poo. What happens is as a fish tank ages it grows healthy bacteria that converts the ammonia to a less toxic form so that the fish can survive...this is what cycling is. It takes time for the bacteria to show up, and theres not any in your tank yet It's best to cycle your tank before you add fish, but the people at the fish store never tell you that because they want you to spend your money buying fish.
What is happening to your fish right now is that the ammonia level is building up in the tank and they are being poisoned. By doing a pwc (water change) you will lower the amount of ammonia so it makes the water less toxic.
Since you just bought the fish, I would STRONGLY suggest you try to take them back to the store and do what's called a fishless cycle. That way, when you buy more fish the tank will be safe for them.
Let me know when you get the pwc done, and I'll give you some more info.