Garter snakes, marcianus in particular, are extremely well suited to naturalistic vivaria. They tend to sit out in the open under the lights, being diurnal for the most part. I would go with the largest tank, in order to allow them to display as much natural behavior as possible.

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Brad Chambers
WWW.HCU-TX.ORG

Boas, Pythons, Colubrids, Tortoises and Turtles