If you have no ammonia, nitrates and no nitrites, then your tank is not fully cycled, thats why the shrimp died. And the dead shrimp are not hte biggest of your problems, because you have 2 common plecos in a 20 gallon, thats
bad. They will get over 20 inches long each, and are gigantic waste producers. You either need to buy a 150 gallon tank with excellent filtration, or get rid of them ASAP. What kind of glass fish do you have? Because if they are glass catfish, you need to get them a larger school. Heres a link to the catfish:
http://www.e-aquarium.com.au/glass_catfish.htm