This thing is awesome! Really simple design but it does the job well it looks like. :) When you have all of the net pots full with lettuce how often do you change the nutrients?

Very nice, simple yet elegant design. I may make one of these myself. Thank you so much for posting this.
I wonder about lighting though- would the bottom heads of lettuce get enough light?

Lighting has always been my question with vertical units. So I am curious as well. I just don't see two CFL's getting enough light down 5 feet.
Also, how do you control the volume of the pump so it doesn't overflow at the top?
Do you have any issues with algae growing where light leaks through at the net pots?
It's a nice clean system, but those are just some issues I see with it.
Regarding your pH issue, are you have any evaporation issues?

The 2 lights above are 400watt metal halide. The bottom plants are doing fine so far. I can't wait to try it in sunlight this summer. At the top I just drilled enough holes to handle the flow from the pump. No algae so far. As for evaporation I'm not sure. Does too much oxygen cause a rise in PH? That's the only thing I can think of.

I don't believe so. the water will only take so much oxygen before it is saturated. If your nutrient concentration is increasing (evaporation) or decreasing (salts being removed) that would affect the pH.