3" roundworms from a 3 week old chick? Something doesn't sound right. Are you sure they are 3 weeks old? Did you take a picture of these worms?
But you could have made a medicated mash with Fenbendazole instead of trying to dose in the water with it.
Redish stool sounds like coccidiosis. Corid...
1 drop per lb of bodyweight if your dropper drops the correct size drop. Some droppers will drop a big drop.
To test, drop 20 drops and see if it equals 1 ml, or 100 drops equals 5 ml, or so fort. If it does then that should be correct. Make sure you weigh each chicken.
I would assume both of those brands have the correct amount of niacin for ducks and not add any supplements. But if you do, any excess B vitamins get expelled. Just off the top of my head I thought brewers yeast was high in protein.
Mix 1/3 tsp of sodium chloride (table salt) and 8 drops of 7.5% sodium hypochlorite (bleach) per cup of water and flush the eye out with that solution as eye drops or flush. That will take care of the pinkeye.
I don't find much info on chickens or if it is safe to use but I see where a parrot was treated for an enterococcus infection with pradofloxacin at 15 mg/kg per day for 20 days. That is similar to the enrofloxacin dose of 15 mg/kg every 12 to 24 hours for chickens. With that information I would...
I'm thinking most feeds are mostly either wheat or corn but yours appears to be green peas. That must be the difference. How much does a 50# bag of that feed cost?
Can you post a pic of the feed nutrient tag so we can see what the feed contains?
@Kiki may be interested in knowing what it contains too for her spreadsheet.
This is the same composition as Reeses (50 mg/ml). Nemex is far weaker and comes out way more expensive. Pyrantel Pamoate only works on the large roundworm.