My flocks not well - advice please

We started with 5 hens back in Feb which we got from a well known breeder, these were all fine and well but the egg production was very low, so recently we have been given 10 more hens from another allotment holder.
All was fine for the first week or so but now we are only getting 6 eggs per day from 15 hens !!
I have had a good look inside the 2 coops that we have and it looks like we have red mite, I have also checked the hens and a couple have mites on their skin.
Please could someone tell me what powder/spray I need to buy without a huge list to choose from.
I just want to be able to treat the birds and the coops without having to destroy eggs for the next few days etc.
Thanks