So i bought a couple of emerald crabs 2 days ago to help get rid a small bubble algae problem and this morning i found them both upside down and dead. I have a 20L reef with sps Lps and softies along with a cherub angel and a pair of mated filefish. My inverts consist of 2 porcelain crabs, a skunk cleaner shrimp, 5 hermits and about 8 snails of different types. All my parameters are in check and everything else in the tank is doing great. Is there any other reasons for the crabs quick deaths other than maybe I just got a couple that were in bad health?