Well, imho a 10 Gallon is sufficient for a 30" snake.
I'm sure they almost always like more space if they can have it, but I'm not sure it can be justified to claim a 30" kingsnake is outgrowing a 20 gallon aquarium.
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3.0 WC; 0.1 CB L. getula californiae
0.1 CB L. pyromelana pyromelana
0.1 WC; 10 eggs (7/11) Elgaria multicarinata multicarinata