The cryptic zone in my reef system is based on the study and writing of Steve Tyree, and uses a system which he calls the "Environmental Gradient" system.
The Environmental Gradient is the name for a completely natural method of captive reef tank filtration. Primary filtration of dissolved organics and pelagic bacteria within this new filtration method is performed by living sponges and sea squirts. Berlin style reef aquaria utilize a protein skimmer to remove dissolved organics, but lack any natural method to control pelagic bacterial densities. The use of living sponges and sea squirts presents a much more natural method to maintain captive reef aquaria. Additional advantages of this new method are its low operational costs combined with the inherent ability to maintain a more diverse assemblage of macro organisms.
Essentially, this area will accommodate sponges and sea squirts which play a role in filtration by consuming
- Pelagic (free floating) bacteria
- Dissolved organics that are not attracted to water-air interfaces, and hence cant be skimmed out
The benefits are clearer water and potential reduction of pathogenic bacteria that can cause coral disease.
Key to the design is darkness (<1% of surface luminosity) and very low flow.
I believe Mr Tyree runs some of his aquaria without skimmers at all, based on this method
I intend to construct it a bit like a bee hive, with substrate "trays" I can lift in and out. From time to time I will be able to lift these out and give angel fish a munch on their preferred diet.
As to sizing, no clear guidelines have been given. A bit like lit-refugia, the bigger the better. Some of Steve's research systems used "sponge scrubbers" that were 1/3 - 1/2 the size of the main tank. I do not have the space, so 160G was all I could accommodate, which is only ~6% of my system size.
This tank will be the only tank without a sand bed. At the low flows required, a sand bed could eat up all the oxygen and introduce particulates that would clog the pores of sponges.