Since this method of filtration is rather new in our forum, i would just like to share this "Natural " filtration method which reefer might find useful and interesting to discuss about this. Below article written by :John Cunningham ( Link : http://www.aquaristsonline.com/blog/general/aquarium-filtration/cryptic-zones-another-filtration-possibility/ ) Cryptic Zones – Another Filtration Possibility There is a scientific term used which is called ‘the gradient concept’.