I felt that the discussion of Oliver's article distracted from his post. To be fair I split off the discussion on dissolved oxygen levels. See Aquaponics 101 Part Three: System Design, Continue for the original post...... Badflash
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20% of 240 GPH to jet back is 48 GPH not 40 GPH, but we know what you meant!

Metal isn't intrinsically toxic, but I know you meant that it will reach toxic concentrations with that much metal surface area dissolving into the water. Remember, some people put iron nails and the like into their grow bed to provide the essential trace minerals for the plants, fish, and microorganisms.

The DO is not really helped by air bubbles, per se. You get far more gas exchange at the surface from agitating the water. A simple PVC with many small holes pumping that 20% return water over the surface will give you a ton more DO. Tilapia only need about 4 ppm DO. As far as the growth performance, that depends on so many variables. To say that the growth rate doubled form below 3 ppm DO to 6 ppm DO sounds very misleading. The pH is more important, as considering that low of a DO given the correlation to all the other factors involved, I really question causality you are implying in your statement. Anyway, I digress... Of course, the bubbles will agitate the surface, but I don't recommend spending money on a fancy air pump or air stone when a simple one will yield the same results. Making use of the return water in an efficient way will provide more than enough DO. I understand not wanting a single point of failure. A cheap $20 air pump with a few air stones will be enough to agitate the surface. A water wheel would be the best option, but I'm not sure how much those run these days. Heck, get a 2nd water pump!

So far, this has been my favorite section in the series, lots of practical information here!