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Photos of the birds, housing and poop?
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Corid drench instructions are in the link posted below. The drench needs to be given in addition to the birds drinking the mixed corid water. If they are not drinking, then you will need to syringe or tube the mixed water into them frequently.
Make sure you are giving the correct amount in your mixed water.
Liquid Corid dose is 2tsp or Powdered Corid Dose is 1 1/2tsp per gallon of water given for 5-7 days as the only source of drinking water.
Do not add any extra vitamins/electrolytes that contain B1(Thiamine) to food or water during the course of treatment.
If you are giving the Corid correctly and they don't improve within a short period with the additional drench, then use a sulfa antibiotic or Toltrazuril to treat them.
Getting a fecal float is a good idea to rule out worms as part of the problem as well.
CORID DRENCH INSTRUCTIONS:
https://www.backyardchickens.com/articles/corid-amprolium-oral-drench-instructions.74391/
