Well, since you're posting in the lizard forum, I can't suggest what immediately came to mind, about six different snake species. Anyways, If it is a 30 gallon long, I'm thinking that some leopard geckos might do well, or perhaps a blue tongue skink, but I can't be too sure on that one, I don't know if a blue tongue will outgrow a 30 gallon.
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0.1 Splendida Kingsnake
1.0 Ball Python
1.0 Sinaloan Milksnake
1.0 Kenyan Sand Boa
1.0 Mexican Black King
0.1 Rough Scale Sand Boa