Most likely, you followed the advice of a fish store employee. Which means you setup your tank, let it run overnight, and then added fish. So, what happened is the fish production of waste has created a toxic environment. Normally, bacteria would process those wastes. There's a sticky thread in this forum about cycling with Fish: What Now? ( http://aquariacentral.com/forums/showthread.php?s=&threadid=3194) that you might want to check out.
And don't feel bad--fish store employees are sometimes not the best source of information.
go to the begianer's faq here. Also there are many other good information sites out there on fish keeping, just search them out. There are books out there to help as well, one good book for begianer's is: "Aquarium For Dummies". Good luck and have fun,
mt_marcy!