Do you have photos of the birds and their droppings?
I'd make sure to keep the water stations clean and see that they are drinking the mixed Corid water really well. Dosing is below in case you need it. Direct oral dosing can be given 2X a day for 3 days, this is in addition to them drinking the mixed Corid water.
Coccidiosis is caused by Coccidia which is a protozoa. There are 9 strains that affect poultry and they are found in poop and in the soil. There is no way to eliminate Coccidia from the environment, so trying to disinfect is futile. Birds can build resistance to the strains found in their environment through exposure, but it's important to treat them right away when there's an overload.
IF you are seeing no improvement, then I'd get some Toltrazuril ordered and continue with the Corid until that arrives.
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.
Direct Oral Drench instructions.
https://www.backyardchickens.com/articles/corid-amprolium-oral-drench-instructions.74391/
Toltrazuril.
https://jedds.com/products/endocox-powder?_pos=1&_sid=8d6293c29&_ss=r