Tank-the 20 gallon is perfect, make sure to get an escape proof lid, even if you think its escape proof enough, make it more. i recomment either reptariums or a really big rubbermade container.
Substrate-i used shredded newspaper, it gives the snakes a sense of protection, doesnt harbor pest insects, and has no impaction risks. if you dont feed the snake on the substrate(i assume you have water for the fish to swim in and a food bowl for the other food) you can use pretty much anything you want. make sure not to use anything sharp(it can get stuck in them and wouldnt be very comfortable to live on, think having to live on mulch all your life) or too wet(they r very prone to blister disease)
Basking Light-i have a 100 watt bulb over my reptarium, it depends where the snake is from what temp you need it at(obviously reptiles from florida would like warmer basking spots that reptiles from ohio)
Water and Food Bowls-a bowl of water to swim in/hunt for fish and a food bowl(mine like bigger food bowls that they can sit in to hunt)
UTH-this would b a benefit, but what i do is keep a smooth rock underneath the lightbulb and this serves as the underside heat element(and its alot cheaper!)
good luck