Hydrofarm Active Air Pre-Filter for ACCF398, 8" x 39"Extend the effective life of your Active Air carbon filter with a corresponding Active Air pre filter. These pre filters block larger particulate matter such as dust and dirt from clogging the working surface of your filter. This keeps your carbon filter operating at top efficiency for much longer. When the pre filter becomes visibly dirty and discolored after several weeks, simply replace it. Check our large collection and find the size that matches