The size of your enclosure is acceptable. I would however replace the sand with cypress mulch or some other type of substrate that holds moisture. Feeder fish are a good prey item and are nutrionally sufficient for waters. You might want to increase the basking spot however to about 85 or so. Keep in mind that even though they are aquatic, they will bask at regular intervals. If the substrate gets too damp blisters on the underbelly may occur. Just provide a large bowl for drinking and soaking. In most cases if the snake gets dehydrated it will often soak in the bowl. I wish you the best of luck and keep us posted on your progress.
-HAPPY HERPING