Do you have data to indicate the rate of diffusion of oxygen in water or aqueous solutions is slow enough to create "zones" in your water column that are low enough to kill your crop or reduce your yields? It seems counter-intuitive to me. I ask you, please heavily question your assumptions.

4 ppm (4 mg L^-1) of dissolved oxygen is recommended by Dr. Wilson Lennard; however, scientific research clearly shows that fish like Tilapia are capable of surviving and growing in waters with less than 1 ppm DO levels. Depending on the temperature, most backyard systems will not drop below 2 ppm DO, and usually not below 4 ppm DO. Unless you are running a high density fish stocking regime in a commercial environment, why the heck would you even bother making your system more complicated, more costly to build, and more costly to maintain? Aquaponics as a solution for world hunger would dictate that poor folks in 3rd world countries would neither have access to or the means to add this kind of equipment, yet many organizations are designing systems that work for them. Rightly so! I understand wanting to have the BIGGEST, SHINIEST, and MOST EXPENSIVE system, but it's just not needed. Please be sure to indicate to people that you add all the fancy bells and whistles because YOU WANT to NOT BECAUSE THEY HAVE TO.

With that said, there are SO many factors that can contribute to the overall performance of your system. Understanding the science and technologies involved will allow a person to troubleshoot or diagnose specific problems to their very unique system and production goals. I can easily give you a context where the information below would really not help, but I think it puts my point on dissolved oxygen into perspective.

Several tilapias are reported to tolerate oxygen levels of 0.1 – 0.5 mg/l (O. mossambicus, 0.1 mg/l, Maruyama, 1958; O. niloticus, Magid and Babiker, 1975; O. mossambicus 0.4 mg/l at 30C and 0.6 mg/l at 35C in closed respirometers - Mohammed and Kutty, 1982; Tilapia guineensis and Sarotherodon melanotheron and O. niloticus - nil oxygen, water depleted of oxygen by a addition of tobacco waste in low concentrations, but fish allowed access to air - Kutty, unpublished).


REFERENCES
Magid, A. and M. M. Babiker. 1975. Oxygen consumption and respiratory behaviour of three Nile fishes. Hydrobiologia. 46 : 359 – 367.

Maruyama T., 1958. An observation on Tilapia mossambica in ponds referring to the diurnal movement with temperature change. Bulletin of the Freshwater Fisheries Research Laboratory, Tokyo . 26(1): 11-19.

Mohamed, M. P. and M. N. Kutty. 1987. Observations on low ambient oxygen tolerance in some freshwater teleosts. Experimentia, 129 – 138.