Is it possible to get a dodgy batch of fish?

I apologize if i'm not wording this very well but i have recently started a small tropical aquarium, i bought 2 golden barbs and 2 tetra's from one pet store, then a day later went to another pet store and bought 2 X-ay tetra's, after about a week and a half we fount one of the X-ray tetras dead tangled in one of the plants, we umed he'd just got stuck and suffocated and that it was an unfortunate accident so we went back to the same store and got another X-ray tetra from the same tank, this didn't look very lively at all and we even had trouble getting him out of the bag, he then deteriorated as the day went on and died over night, and then the day after that my last remaining X-ray tetra ped away, my other fish seem fine but the 3 X-ray tetra's from the same tank have all died, i've had a water test done at the pet store and they said it was fine, what could of gone wrong?