I don't think that "things you can buy" or "things you can dose chickens or coops with" is a very sound approach to preventing health problems.
To my way of thinking, the very most important two preventive measures are 1) practice good biosecurity when acquiring ANY birds, meaning preferably you would only buy day-old chicks or hatching eggs and only from a good clean source (no auction or swap birds, no part-grown birds, no adult birds), and quarantine as appropriate; and 2) keep them in as good conditions as you can manage, which means doing everything you can to minimize crowding and social stress, keep the coop and run reasonably hygeinic (from a functional standpoint, not like 'you can eat off it'), healthy feeding, never letting them run out of food or clean water, excellent air quality through appropriate ventilation, etc etc.
No dust or additive is going to make up for problems created by shortcomings in the above two categories.
Good luck, have fun,
Pat