fish continue to die?

Hi I have a 55 gallon aquarium that has been set up and running for almost 4 weeks now. I have now 2 neon tetras, 3 baby jelly bean parrot fish about an inch in size each, and 4 fancy guppies 2 male and 2 females , and 1  4" pleco. I had three more fancy guppies and a black moore  but they all...