By the time the weather warms up, I order Dutchies, the natural enemy of mites and put those in the coop. They actively seek out the mites and eat them (and some other pests as well). And I use diatomic earth in their sand bath. No need for chemicals, no chemical traces in the eggs and no danger...
In the introduction it is stated that you don't know where the Polish breed came from. They most certainly bare a striking resemblance to an old Dutch breed, called Hollands Kuifhoen (Dutch Crested Chickens), and they have been documented in paintings in the 17th century. However, the assumption...
Being that you are not in the US but in Europe, it also could be a disease caused by a parasite called coccidiosis. At least on the continent it is a prevailing chicken disease, but there is medication for it.Available at the vet's.
However: it is highly contagious, so any lethargic, puffed up...