Mice stink when you set them up on BAD BEDDING. Aspen or any other wood shavings is bad for rodent breeding as it causes bad smells and elevated ammonia level in which can cause sick feeders.
Pine pellets is not your average pine as its biodegradable and absorbs their pee and breaks down into a sawdust.
Just hate when ppl come in ans say mice stink when i been breeding them for a while and never had a problem with smell and i have 500+ mice not including rats or babies.