black fantail fish

Question
I bought a new black fantail fish last friday. It died yesterday. Whats gone wrong do you think?

Answer
It could be one of several things.  Too small a tank (goldfish need a minimum of 20g each, even when small), Chlorine in the water, uncylced tank (toxic chemicals build up with no bacteria to dispose of it), shock from the transfer to a new tank, or just bad stock.  
Here's a link to a site with a great e-book for beginners.
www.aquaticcommunity.com
I hope this helps.