Oh Min, I hate those red mites. Be very vigilant about checking for the next couple of months, especially at night with a flashlight. I found it very hard to get rid of them. Ultimately I had to get a flea spray meant for pet beds and spray the inside of the coop (not the chickens). In fact, now that the weather us warming up, I'm going to spray again. The mites come in on wild birds.
something with spinosad sprayed in the coop. It listed as being non toxic to chickens but will kill bugs. It persists for a long time when not in the sun so it would be perfect for coop nooks and crannies.
Something like this:
http://www.amazon.com/dp/B001VJBYMC...lid=1UZY82ZFXAAHB&coliid=I3CTFU3C8PPSLU&psc=1