Manufacturer's rating can only be taken with a pitch of salt, moreover your skimmer is china brand which makes it even more difficult to trust the rating
Job of skimmer is to remove DOC before it has a chance to break-down and that results in NO3. Your 30ppm NO3 tells me your skimmer not enough juice
As for water flow difficult to advise you, we already asked for detail setup, equipments etc and photos if possible.
Try to wash your floss or filters on a weekly basis, that will help too.
See my previous post for some other reasons.