You can scroll to below posts for more info. Basically fresh fish have an amino acid (thiaminase) that destroys thiamin (AKA vitamin B) in snakes. The result is a neurological deterioration that will eventually kill the snake. They start to loose control of their movements, typically curling over backwards when they intended to crawl forward. Its a pretty ugly disorder I think.
Its not that a few fish will kill the snake, it can take months of a steady diet of fish and nothing else. In the wild this is probably very uncommon, but in captivity its not uncommon. You can buy a good multivitamin designed specifically for herps that will supply thiamin. Feeding a variety of prey items is another solution. As Pierson pointed out below, freezing the fish does NOT help, heating them up might though.
The other important considerations for keeping most herps apply: A temperature gradient created with an overhead light or other heating element, clean water, a hide box and climbing branches.
Good luck,
Joe