Well, a few words of caution. Large emerald crabs can attack small fish. I think that's happening in my tank.
I bought six emeralds to help control a bad algae problem. The algae's now gone and some of the crabs have grown to about 3" across including claws. Several small fish (chromis, scooter blemmy, neon goby) have disappeared without a trace. The other day I found a chromis with a nasty gouge stuck on a powerhead intake.
I regularly test my water, and it's fine. Meanwhile, I've seen the crabs try to grab passing fish.
I'm going to try and get the critters out of my tank this weekend, which will mean taking out all of the rockwork.
Riclat