Why do my SHRIMP DIE?

HELP me before I kill my new fire shrimp. I have tried three different kinds on shrimp of my biocube. A peppermint shrimp, a coral banded shrimp and the last fatality was a cleaner shrimp. I have bought one shrimp a month from 3 different pet stores thinking that was the problem. My fish in the tank are fine and healthy. I have 2 clown fish and lawnmower bleeney. and probably 25-30 pounds of live rock. I dont get it they are supposed to be easy to take care of. Mine don't make it passed day two. My nitrate level is slightly elevated would that kill them?   HELP

 

We keep the sallinity at 1.020-1.021 a little low but I heard it is bettter to keep it a little low in a quarintine tank. The temp is gone up a little bit since it started getting hot outside it has been between 78-80 degrees. I did a slow drip acclimation for a hour. I had ich about 4 months ago and used ich attack on that tank. Petland is a great store and Brett knows his stuff. I recently bought my clown trigger from them a couple weeks ago and "spot" is doing great. He is happy and healthy. Do I get a discount now Brett? LOL

 

Thanks for the advise. I will test the tanks copper level. I didn't know that ich tx would absorb in my live rock. Has anyone tried using cycle before adding new fish or shrimp? Is it worth a try to prevent livestock loss??  Thanks