5 gallon for something

any sort of terrestial taruntula will do pretty much, I'd reccomend a mexican red-kneed.
Tarantulas are kind of agoraphobic, so they like smaller cages.

A hermit crab or two would work as well.
A house gecko MIGHT work as well, but I don't know much about them.

 

I didn't know you could still get mexican red-kneed tarantulas. They're on the CITES list as far as I know. I say an african dwarf frog as well. They're Cute!

 

im not all that good with differnt species but i kno a ten gallon tank is only 10 dollars. i dont think there is a animal in the entire world that would do good in a small 5 gallon-hex tank. no ofense or anything but if ur that stuck on money that ur worried about throwing out that little tank and spending only a few extra dollars then maybe at this time a pet is not right for you. the only thing i could even think of is fish or a normal house spider. lol.