Why did my fish die in 3 hours? (Fish bought from walmart)

I've never had luck with any fish I bought there. I bought three small baby goldfish from there and put them in a big tank. They were bad at first but the first batch died after three hours. Lucky, I was able to return them but it was kind of annoying. I kill fish but not that quickly. I did that two more times before the fish person at Walmart told me that the ammonia from the tank was probably what killed them. The next day, a tank that had twenty gold fish in it was down to two small ones, that died soon after. Since then, I've always noticed that the fish never look healthy and that they always seem over crowded for their tanks or empty.