Betta Fish Died Any Idea Why?

Ok so i got a Betta fish and it died within a few hours of being put into his tank. I have a 2 gallon tank I rinsed the gravel put it in then put water into the tank dechlorinated the water and let it set overnight to equalize temperature. The tank has a filter. Then within a few hours of putting the Betta in was dead. Any ideas what went wrong? I noticed awhile before he died that his eyes seemed to be cloudy. Also, he was acting lethargic. Thanks for your help.