From what little information we have, I imagine its something to do with the cycle of the aquarium.
they should generally be cycled before fish are added. It's kind of like dropping you off in death valley with a bag of chips and assuming you will be fine.
All you can really do is change the water a lot in an attempt to neutralize the waste products before they damage the fish too much. until a cycle is established.
take some water out every day and replace it with new dechlorinated water.
so essentially you need to establish a colony of bacteria that will eat the waste product of your fish and neutralize any harmful effects. this is called the nitrogen cycle. once the cycle is established, the fish should live in harmony with their environment.
You can buy some culture in a bottle products that contain the bacteria to establish your colony from most LFS. that is another option.
your cycle probably isn't established yet you can get beneficial bacteria and other products at local pet/fish stores and you should also be using a conditioner for the water if you used tap water to fill the tank. I recommend getting a water testing kit to see what the problem is