My first snake was a kingsnake, my second was a KSB. They're both great snakes! A kingsnake might eventually outgrow a 20 long, depending on which species you get. Aspen bedding is a nice substrate for both of them, many kingsnakes like to burrow.
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"The serpent crams itself with animal life that is often warm and vibrant, to prolong an existence in which we detect no joy and no emotion. It reveals the depth to which evolution can sink when it takes the downward path and strips animals to the irreducible minimum able to perpetuate a predatory life in its naked horror."
Alexander Skutch