My fish keep dying

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Ok I need help. I bought 4 Beautiful tropic fish for my daughter. We did everything the pet store told us. Fed them perfectly on time every single day. One died two days after we got him. Two died 2 days later. And the last one died today! I have a 5 gallon tank that has a filter. After the first one died i called the pet store they told me to turn off the filter it was to strong. Did that and 2 died. Called the pet store again told me to put the last one in warmer water cause they apparently need warm water. So i did that Two days later he died. Can anyone tell me what im doing wrong. Also if anyone knwos what fish live the longest and are the best to buy would be greatly appreciated. My daughter is upset that her fishies died. Shes only three and i really want to make her happy!