Every time I add a new fish to my aquarium it dies but the fish I've had in there from the beginning are perfectly fine. Why is this?

maybe stress

your other fish might be chasing them or eating their fins, also your fish are use to the water conditions in your tank, they might have diseases from the store and you might not be transporting them right First its important to remember that each type of fish requires a different water PH and even prefer a certain level of the tank from bottom, middle to top. Does your tank allow this? Are you mixing aggressive fish with less aggressive, big with small? You should always dechlorinate your water when adding new fish and take the acclimation process serious and let them adjust to the water for no less than 15 minutes (in the bag), this will also hint at how the other fish are going to react. Like the writer above wrote diseases are very prevalent in your typical fish store because getting your fish from the sea (yes, they still come from divers--especially saltwater) us a LOOONG process for a fish. I have worked at a fish store and those gallon size bags their delivered to the store in are atrocious! Full of feces and could you imagine the quality of the water. Many workers trying to be quick will mix a lot of the same water into the store water and this spreads disease and parasites.the other fish eat them