2 tsp per gallon. Make fresh daily and treat for 5 days as the only source of water.
If you have any chickens showing stronger signs of coccidiosis, drench her directly at a rate of 1/2 tsp undiluted corid per 5 pounds body weight for the first 3 days then allow her access to the corid water for the last 2 days.
This is how I did a treatment when one pullet went suddenly lethargic with puffed feathers about 2 weeks after new pullets arrived on the property. She looked totally normal after 1 drench.
Here is the correct dosing for Corid, for both powder and liquid forms, use the severe outbreak dosing.
The oral drench dosing is here: https://www.backyardchickens.com/threads/corid-oral-drench-instructions.1211991/
and the drench should be given to any birds that are showing symptoms, for up to 3 days in addition to the medicated water. Make sure the medicated water is the only water they have access to. If a bird is not drinking well, you can give the medicated water orally several times a day until they start drinking on their own. They have to take the medication in for it to work. Corid is very safe to use and over dosing would be really difficult to do. Don't give vitamin supplements during treatment as supplementing thiamine could reduce the effectiveness of the medication.
You have not given much information about your chicken, such as age, how it is acting, if it has been eating, etc. You have just asked about Corid. Chickens can have runny poops for other reasons than coccidiosis. Corid can be used at 1/3 the dosage for another 5 days safely.