X2Justine..I would take a fecal sample into the vets office.
Treating without knowing is counter productive sometimes. When you are working with this number of chicks, you need a definitive answer to get you going in the correct direction.
I had bad results in the past treating without knowing. Lost many many chicks and became so so very frustrated.
These days I know more about preventing than treating.
I have walked in your shoes Justine and I know exactly what you're feeling right now. You will get a handle on this outbreak, whatever it is. I have utmost confidence in your abilities.