I tried searching for some pictures. The small crabs that I've encountered in aquariums that looked like emerald crabs, but were orange with orange or blue/black legs. Were I believe from asian mangrove areas. Actually need to be able to climb out of the water from time to time, need dirtier water, less light, and are best suited for a species only tank. Sadly, I couldn't find any pictures online to show you, and let you tell if thats what you have.