Why did my fish die ?

I bought several fish at the pet store a few days ago. Everything had been going well until one of my fish died. It was a Red Wag Platie and it was with other recommended fish, plants, gravel, etc. I did notice that it wasn't eating and was actually laying upright on top of the gravel. It wouldn't swim around either. It didn't have anything on it nor did it look strange, in fact it looked quite healthy and beautiful. Do I have anything to worry about for my fish, and if so what should I do to prevent this from happening to another of my fish?