Avoid the rosy minnows. Those and goldfish are saturated with thiaminase (a bad thing). Once in a while probably wont hurt and other members often start stubborn feeding babies on them. Still, best to avoid them. Try guppies or mollies. I'm willing to bet your snake doesn't even need live fish at all. He'll probably take f/t and that would be better anyway.

http://www.ces.ncsu.edu/gaston/Pests...gsnakecare.pdf