How long has the aquarium been running? If it's a new tank, it's probably "new tank syndrome". When you put fresh water into a new tank, there is no bacteria in the water. You need that beneficial bacteria in order to have a proper nitrogen cycle. There is a post on the main "Other Pets and Animals" page about the cycle, so check that out for a more thorough explanation.
Basically, when you add a bunch of new fish, you're filling your bio-load with ammonia--the leading fish killer. The only way to counter that ammonia boost is to have an established bacterial supplement in the water-- on your decorations plants, and gravel. This is the environment needed for the nitrogen cycle.
You can buy a generic "bacteria supplement" or "Stress Zyme" from the pet store to add to the water to help, but it'll take time to establish. Make sure to follow the directions on the bottles as well because you can do damage to the water by overdosing.
You'll probably continue to lose fish in the process, so don't buy any more until everything's okay. Invest in a "six-in-one" testing kit to measure your water parameters, or take water samples to local pet stores and ask if they can test it for you-- most will.
Good luck.
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