We decided to make the Biofilter separate from the RDF so that we can make it 2-3 times bigger (in volume) than the RDF. That will provide a large volume for the biofilter that can handle the fish load. Also by doing this, the RDF can be nested inside the bio box to save on the shipping cost.
The total volume inside of the Bio Filter is 20 cubic feet so we can load it with 10-15 cubic feet of K1 moving bed media or 15-20 cubic feet of ceramic media such as lotus media by Mountain Tree.
We are highly recommend to purchase the Bio box along with the RDF to save on shipping due to the oversize of this box.