items are perfect for my 18g shop vac. Filter requirements depending on the manufacturer are shown in the photo. The kit contains 2 fiber filters and 4 foam filters. The foam filters are placed on the rack under the fiber filter. I currently have both foam and fiber filters and they work satisfactorily. It's probably unnecessary to start both, but so far no damage and no foul. My intention is to remove the fibrous filter for wet ops and leave the foam filter in place.