Interesting question SBTR. I think we are close to Lord Flynn and In-the-Hills in many respects. In our case it certainly isn't a fundamental policy, but simple trial and error. Never used Diatom (heard it helps but is expensive) but certainly wouldn't use it internally. Tried ACV but saw no benefit -certainly didn't work on worms even used once a month, probably because it needs 3% and at that it is totally unpalatable. We put no additives in water, just give them good quality food (no chemical colourants, yolk enhancers they call them), fresh fruit and veg when available and grass. The only wormer I trust is Flubenvet, having been lured into trying Verm-X (which has some effect so could be used in-between). We avoid Ivermectin because there are many alternatives which just take more effort. We use onions to control Cankers -with limited success. We do prevention and cure, focussing on maintaining a stress free environment. We haven't seen a vet (apart from export) for years and our vet learned a lot from our weird cases. As we check poos daily, and talk to our birds a lot (well I do) problems are spotted immediately, so we don't get that many problems (although Carbon Monoxide poisoning took a long time to figure out). Potash works well for lice and perhaps most mites but isn't much good with red mite -experimented recently and concluded creosote is the only way, or a plastic coop. We rarely cull, or need to, because we spot problems early (generally). Just accidents or meat birds. The key to keeping your hens healthy I think (apart from good husbandry) is minimising stress- just like humans really.