You could try the old cigarette trick: hold a burning cigarette or cigar beneath the mite-infested areas, the smoke will cause them to suffocate. As long as you are careful not to drop ashes in the water and you use a fan to blow the smoke out after treatment, it should not affect your aquatics. *CAUTIONARY NOTE* I have never tried this in a terrarium. In fact I haven't done it at all (I'm not a smoker), but I have seen my grandparents successfully eradicate spider mites from houseplants using this method.