Folks, if you don't know what is wrong with your chicken, it impossible to effectively treat your chicken.
There are many types of illnesses and infections. They require different antibiotics.
I had a finch recently who was breathing hard. I called the vet and asked if I could buy some Erythromycin or Tetracycline at the pet store. The vet said 'no, don't do that', and asked me to bring him a poop sample and I did. He saw bacteria in the stool that could only be treated with a sulfa drug. So I bought some from him and treated the finch. She got better right away.
If I had bought the pet store antibiotic and put it in her water, it would not have worked and she would have died. I recommend having a vet that you work with. You don't have to bring the chicken in every time, but once a vet gets to know you and your flock, you can call and get directions for treating a chicken. There is no one wormer or antibiotic that will help all conditions. You have to know what is wrong first and then use the right medicine to treat it.
I am sorry for you loss! God bless you.