need some help with wild caught yellowbelly watersnake

[yellowbelly2001]

Im from far south dallas. I live on 100 acres and am 99% sure this morning i caught a yellow bellied watersnake. Hes very pretty, ive never cared for reptiles and from what i learned, i have him holding in my mouses 20 gallon aquarium, hes chilling in there with her, if he eats her its fine cause i caught her about a week ago and could catcatch another, but what ive read they dont eat mice just fish, im cleaning a 120gal aquarium later will have a rock setup hiding spot 2 inches of water heating lamp and what fish where do i get them and how should i feed? The rock will be long and flat and provide 2x4 feet of coverage thats dry, the rest water. Whats the best way to feed and qill a setup like that work? I care for about 50 different types of animals on the ranch and this is just added too the collection, any help tips or tricks or important info please help, im on a phone. Or id post pics, hes very yellow 3 feet long circle pupils black tongue blunted head greyish greenish blackish on top, his belly is bright neon yellow