=C What did I do wrong?!? Please help me. *Final Update**

Ok heres the story, keep in mind I'm new to chickens and I pretty much hate myself right now.
I have (well had) 4 chicks in the house in a brooder, 2 silkie 1EE and a Buff O. They are around 4-5weeks old. Just two days ago I discovered bloody poop. I immediately thought cocci. I didn't know who had it but I knew it was alot. so figured at least 2 had it. It was evening and couldn't go get the medicine. Yesterday I went to get sulmet at a feed store but they had just sold out. They were the only place I knew at the time that had it. This morning, I checked them and I could tell they were going down hill. I called around at different feed stores and finally found a place that had it. I bought it and came home and followed the directions. It said:;
2 tbl spoons per gallon of water for chickens. So i did that.
I dipped each of their beaks in it cause the directions said if you can treat them individually do so. Doing that, I let them be for a little while. I came back bout 30 mins later and my silkies were on the verge of death...they pretty much died in mine and my moms hands. My white one was throwing up and all gooey...
I feel like I killed them, was it too strong a dose and it killed them? Or was it too late to save them? I'm soo sad right now, they were my babies..
My EE is hanging on and I don;t think she'll live much longer. My Buff seems fine, but don't know. Please anyone know what I can do? Or what I did wrong, I feel so terrible!

 

Thanks guys. I feel terrible. Is there anything else I can do fo rmy EE? I took the water out cause I felt it killed my silkies. Should I put it back in?