OK, I would go ahead and treat all your chicks and chickens for coccidosis. Cocci is a fast killer and an easy fix if you catch it soon enough. The treatment will not hurt your birds even if they dont have coccidosis. The treatment is Corid (amprolium). For the powder it is 1 1/2 tsp per gallon of water for 5 days, make a new batch every day and make this their only water source.
If you get Corid 9.6% liquid solution. The dosage is 9.5cc to a gallon of water for five days.
Do not give vitamins while you are giving the treatment. YOu can find Corid at TSC in the cattle section.