Hello! I think the pet store was right. When you brought it home, did you do the thing where you put the tank in the water for 20 minutes first? It's really important that you do that. It's very common in newly purchased fish for that to happen so don't worry about it.
put the tank in the water? lol the bag? But what were your water levels? Ph? Ammonia? Nitrite? Nitrate? Temp? What species was it? How big was it? What tankmates were there?