If you are seeing many mites, it will not hurt to change your substrate. Perhaps you just bought a "bad" bag of substrate. Next time you could try freezing the new bag of substrate or cook it in the oven (this will stink) or microwave for a short while.
I usually just change my substrate if mites becomes a huge problem. Sometimes I wash the larvae or adults with distilled water if I see mites on them. I will even use tweezers to pick them off.
If you have a photo of the mites, this might be helpful in identifying their source.
is it possible to make photos of mites (unless there is some good profi camera with sufficient macro lens) ?
to me they seem very tiny, almost microscopic. I guess my camera is too simple for making good enough close up of them
(only may be if I do some VERY high resolution image and sort of crop them out - sort of macro-zoom trick through high pixels?
perhaps I'll try tomorrow in a daylight)
I don't know about OP, but myself have seen so far 2 kinds of mites:
1) one kind is red color, these are a bit bigger size and I found them blocking the mouth of one of my C. atlas males (perhaps due to it eating fruit jelly all the time).
simply brushing them away didn't work well: they are persistent bastards, keep running away from the brush and then coming back. finally I just used water to wash them off.
after that I haven't seen them, well, at least not in the same area (around mouth).
2) another kind I saw is white color and look like almost half size of those red ones.
these were in the substrate (feed) of my larva, as well as few of them were on larva too.
are these dangerous for larvae ?
I have also noticed some tiny white worms, hardly 1-2mm long. I was told these might be some Nematodes ?
seems like it is quite a tricky business - keep away all these pests and parasites, phew !