Fish being killed by copper banded shrimp??

[Jcs401]

Ok so I have a 24g nano and had 2 clowns 2 firefish, a meandering golby, and wrasse. I bought a copper banded shrimp and I saw it nip at a clown. Next morning woke up and it was dead. Few weeks later the other clown dead. I am now down to only two fish as my fiancé said "the mean shrimp I think killed the mandarin". I asked the store and they said the shrimp is peaceful and would never kill fish. Is this true because I have seen it nip at all of my fish?? PLEASE HELP!