The store might or might not have measured all the toxins in the system. All they had was a vial of water, a test kit, and a customer they want to sell something to.
Take your time and add no more fish until the tank has matured a few months or is showing signs of stability. Signs such as tiny shrimps, small sponges and worms spontaniously appearing. For the first few months, the display will still have little cycles. If the tank was perfect, there wouldn't be dying fish :>(
Sorry if this sounds like a downer