Its pretty common for new fish to die within the first week. There are a lot possible reasons.
Injury or stress from transport. The fish was hurt by a net, breathed its own waste too long in the bag (on the way to or from the store), or was just "scared to death"
Disease. Fish are exposed to diseases from all over in the pet store tank and when you put them in your tank, they get exposed to any disease your fish carry but are immune to.
Ammonia/mini-cycle. New fish always add to the "bioload" of the tank and takes a few days for the filter to catch up. Esp. if you dump the water from the bag in your tank. Ammonia can spike (suddenly rise) and kill your fish. The new fish is vulnerable because it is already stressed, but the old fish can die from this too.
Aggression. Your old fish defend their tank by attacking the newcomer.
In any event, whenever you have a fish die in your tank, do a water change to deal with the ammonia and wait 3 or 4 weeks before getting any new fish. If your fish have been exposed to a new disease, you want to give them time to show symptoms.
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