I'm sorry for your loss, @Dees ducksI'd treat this immediately..keep them dry & warm & give niacin (water soluble vitamin b supplements) & electrolytes. Don't worry, poultry cannot overdose on vitamin b as the excess will transfer to waste & the more electrolyte ingested right away the better.
I know it's too late now, but just to clarify, ducks do need extra niacin, but when treating coccidiosis it is important to know you must not give vitamins. at the same time. Amprolium works by blocking the B vitamins that coccidia need to survive, thus reducing the number to the point that the birds can build up a resistance to the coccidia. If you add vitamins it can negate the effect of the amprolium.