Water snake husbandry

[StudentoReptile]

So I had been looking for a water snake lately, and finally a friend found me one. I normally don't go for WC indigenous species, but I wanted a Nerodia species for educational purposes.

Anyway, I got him set-up and I know that they eat a little more often just weekly. I plan on feeding him mostly amphibians when I can find them and raising my own feeder fish [probably convicts]. Question: how often should I feed him and about how much in one sitting?