These dosages were posted by Casportpony in a past post:
"The 9.6% Corid liquid dose I use is 10ml (2 teaspoons) per gallon for 5 days, then 2.5ml (1/2 teaspoon) per gallon for 7 days. Make fresh daily.
The 20% Corid powder dose I use is 1.5 teaspoons per gallon for 5 days, then 1/2 teaspoon for 7 days. Make fresh daily.
Medicated water must be their only source of water (no creeks, puddles ponds, etc.)"
Don't delay treatment, start as soon as possible. You can also make a drench with the corid to give to an individual bird if it's very sick.
If you have liquid corid, just use a drop or two straight. For the powder you mix 1/2 tsp of powder in 10 ml of water. Dose the bird 0.07 ml of that mixture per 100 grams of weight of the bird, once a day for up to three days. The drench is used in addition to the treatment above, if neccessary.
Good luck, hope your birds get better!
Also, if you have cocci in your soil, this can be a recurring problem. Good idea to keep Corid on hand. New birds will be susceptible, and I sometimes have an outbreak after rainy periods where the ground stays wet. Chicks are more susceptible than older birds, but can happen to any. Corid is very safe to use, and I don't hesitate if I a bird shows signs.