I have a 20 gal and i'm wondering if it might be alright to mix two small house geckos with a hermit crab?
Honestly, it's probably better not to. If the gecko gets decides to rest down on the ground (which they will do if there's some place to hide down there), the crab could very easily grab hold of the gecko's tail (and house geckos will drop their tails)... Aside from habitat condition conflicts, that would be my biggest worry.
