2 Very Calm Nice Red Belly Frogs Big Tank No Friends? =(

[fishychick77]

so i have 2 veryyyyy sweet harmless red belly frogs in a 20 gal terrarium. they used to be with 2 fiddler crabs but they molted and didnt play nice, and ate eachother. my questuon is i have all this room but what else can be with them?
i reallyyyy wanted mudskipper but cant find them at all i called atleast 20 places.
so any idea let me know thanks