People do all kinds of strange things to “help” their chickens, our chickens usually survive and often thrive in spite of it. I have no idea what kind of dosage half a dose once a week works out to be or how much of that mixture they are actually drinking. I assume you are using Corid to get the Amprolium. The dosage in Corid is meant to be a treatment, not a preventative, but at that dosage and way of giving it I have no idea what effect it might have, probably not much. The amount of Amprolium in medicated feed is specifically designed to be a preventative, not a treatment.
Amprolium is not an antibiotic. It is a Thiamine blocker, intended to help reduce the reproduction rate of the protozoa that causes Coccidiosis to a level that allows the chick to develop immunity if that protozoa is present. It does not destroy any probiotics in their system the way an antibiotic would. If that protozoa is not present, it does no good at all.
A little bit on the life cycle of the protozoa may be helpful. The bug lives in the chicken’s digestive system. It reproduces by oocysts that eventually develop into new bugs, sort of like eggs that hatch. It’s not the right terminology but I’ll us use it anyway, I think it’s easier to understand. That bug thrives in wet manure-rich soils, especially warm soils. People on the Gulf Coast are much more at risk than someone in the upper Midwest but it can still show up anywhere. After about two days in warm wet manure-rich soil the egg has developed enough that it will hatch if the chicken eats it.
The problem is not that a few are present, that just lets them build immunity. The problem is when those numbers get out of hand, then it can become deadly. Since it thrives in wet conditions, keeping your brooder, coop, or run dry is usually enough to keep it under control but some strains are stronger than others. You still have to watch for symptoms, whether you feed medicated feed or not or keep your brooder dry or not. Another important preventative is keep the water clean. That bug can thrive in poopy water too.
I do not feed medicated feed, although I know that bug is in my flock. I keep the brooder dry and the water fresh. To introduce the protozoa and keep it present so they can develop immunity I feed them some dirt from my run every few days.
One very common occurrence on this forum is that people that do not understand the life cycle of that bug or even how Amprolium works will feed medicated feed while the chicks are in a sterile brooder and never come into contact with that bug so they can develop immunity. They stop feeding medicated feed when the chicks leave the brooder and hit the ground, coming into contact with that bug. The chicks get sick and people complain about how medicated feed does not work.
If it were me I’d stop putting that stuff in the water. Your circumstances and set-up are different than mine, but I’d feed the regular feed and just watch for problems. The Corid is handy if you see symptoms of Coccidiosis. You can switch to medicated feed if you wish, they will handle the switch to a different feed and save the starter until later. They can eat that non-medicated feed later without harm, just store it in a relatively cool dry place. The freezer would be optimum, but just cool and dry works as long as you don’t wait for several months. It does lose some nutrients the longer it is stored but cool and dry helps it last a lot longer.