Hi Marissa, welcome to FK
Can you tell us what sort of fish they were?
After 2 weeks your tank wouldn't have been ready for fish. Unfortunately you can't trust what most shops tell you
A fishless cycle can take up to 6 weeks and it's best to get your own test kit because a lot of shops will just tell you it's 'fine' without telling you any details. Water quality is very very important to your fish's health. Have a read of this and it will tell you more-
http://www.fishkeeping.co.uk/articles ... hless-cycling-article.htm