Hi Vickichics,
You may want to search the forums for "worming schedules" "parasite prevention" and such.
As far as I can tell there are widely differing opinions about this - what works, what does not, how often to use it - **but** there are common reports on what works for most people.
I am no expert on chickens - we only have Guineas - and I have been going back and forth on how much 'poison' to give them. Never had external mites (so far so good) and never had a problem with internal worms when the vet checked.
Vets, by the way, have given me widely differing opinions, too, on wormers. I err on the side of not treating but that is a personal decision.
We really started treating hard when we got feather or quill mites one year and the birds went downhill fast - I went to the vet, cried, tried a number of things, went to another vet, etc. In the end one vet prescribed the IVOMEC ivermectin pour-on for cattle and we worked on the dose (I made a weighing cage to hang from a suspended scale) and that was the only thing that got rid of the quill mites for us.
I had to treat 3 times, highest recommended dose, 7-10 days apart and when I missed it, I had to start the 3 times over again - arghh!
After all that, only gapeworm could have survived and I treated with the goat wormer (forgot the name but can look it up if you are interested) for that but am not doing that regularly.
Some of our Guineas will be 8 years old this year! The ones that did not make it died from predators, tumors, error in feeding (too many treats in the winter), and 2 unknown (may have been parasites?? but no proof of that).
I use preventative Diamoteceous Earth (DE) **under** the bedding and in crevices where mites tend to hide. I don't use it on open surfaces so the birds don't whirl it up and we all don't breathe it so much, but some folks put it in their dust bath - I don't.
Hope that helps a little.
My advice: ask lots of questions and stay sharp. Use your own mind to think about it and observe your birds. Use what you think is best for you. Best of luck!