How much of the drug are you using in the water? In the link in post 6 it gives several mixing instructions. Basically it says to dissolve 1-2 grams of powder into a liter of water, and treat for 3 days. Skip 2 days, and treat 2 more days if symptoms persist. Of course, that is for treatment of coccidiosis.
There is also a drench where the whole packet is mixed into a liter of water, and given orally. Then it says to give 10 ml per every 10 kg which is 22 pounds. Chickens probably would get 2-3 ml maximum per day.
Can you try to get FishZole online? Look at post 5 for a link. Dosage is 250 mg daily for 5 days.