You can comfortably house both rats and kings in a 20 gallon long aquarium. Care is the same for kings and rats. The biggest problem will be deciding which one you want. As far as care, take a look at the care sheets here on kingsnake.com. If you are unsure of something, ask. There are alot of knowledgeable people on here that will help you.
Be forewarned....snakes are VERY addictive, and once you get one, the 2nd, 3rd, 4th, etc. will not be far behind!! LOL
Welcome to the world of snakes!
My hatchling everglades rat snake.

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1.0 Corn snake
1.0 Black rat snake
1.0 Albino Black rat snake
1.0 Everglades rat snake
0.1 Yellow rat snake
1.1 California king snake
1.0 Prairie king snake
0.1 Black king snake
Good luck and Happy Herping
Brian