Arthritic pea chick

[jjatl]

I had a peachick that hatched and the other eggs mom sat on didn't hatch so it was alone. I brought inside and it was doing great at about 3 wks old it was acting stiff and hurtful to walk. It continued to get worse and died. This morning, I have another peachick that is about 3 wks old and it is walking stiff and tired. Last night it was fine. I read that some can get bacterial infections and get arthritic. I mixed one amoxicillin 500 mg to 1 liter of water. I dont really know if this is the correct antibiotic...also not sure that this is a bacterial infection. If anyone knows what antibiotic to give or know if this could be another illness, I would appreciate any advise and treatments.