It doesn't matter where you get your fish from. Whether it is walmart, petco, petsmart, or the lfs down the street, your fish could have something wrong with them. The best thing to do is quarantine your fish in a seperate tank for 2-3 weeks to make sure they are fine. The longer you wait the better. I had fish that didn't show any signs of disease until 3-1/2 weeks after I got them. The bad thing, was that I added another group of fish I had been observing to the tank and they were infected by the others.