To most people I sound like a broken record but instead of K1 it might be worth looking at Poret Foam (http://www.swisstropicals.com/filtra...ua-media-shop/) as a fluidized media. Stendker discus also use it as the basis for mattenfilters. Think scalable, long-lasting and effective sponge filters on steroids.
Did you decide to go with RO? Overflow design?