PVC Tower System - Adding oxygen mid-stream

[topher_cohen]

Greetings everyone! I am a newbie at Hydroponics although I have been doing research for years. I have a 20' x 20' greenhouse that I am not utilizing to it's fullest and finally pulled the trigger with my first hydro system. I have built my first hydro system from 4" PVC in 8 - 10 foot intervals joined by 90's around an 8' 4x4 with a 50:1 slope on my run. The water is oxygenated in a resevoir and pumped to the top. I have read that one of the downsides to this method is that the water loses its oxygen before it reaches the lower plants.

My question is, is there a reason I shouldn't add an air stone in the 4th set of 90's to add to or replenish the oxygen for the lower plants? It makes sense to me, but I am too new to see the downsides. It seems like if that was the solution, others would have done it before me. Thanks!