Correct dosing for Corid is on the chart below, use the 'severe' dosing on the right side of the chart. I would start and do a full round of treatment right away. I am guessing that your one dose was coincidental to the onset of symptoms. That is a lot of blood in the dropping on the floor, and I see more in the brooder on the heat plate. You can also give an oral dose of the Corid water to any chicks that you know are passing blood, or any that seem off or lethargic at all, it's important to get the meds in them. Give the oral dose
in addition to the medicated water, dosing for that is here, you can give the oral dose for up to 3 days:
https://www.backyardchickens.com/threads/corid-oral-drench-instructions.1211991/
Keep feeders and waterers as clean as possible and keep droppings cleaned up, the droppings will spread the illness. Also make sure bedding is not wet or damp anywhere, that also contributes to the illness. I would start treatment right away. The treatment is very safe, you don't need to worry about it doing any harm, the illness can be deadly quickly. No vitamin supplementing until treatment is complete as that can reduce the effectiveness of the medication.
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Thank you so much! I was panicking