queenkat, I'm going to take a stab at this as I've been researching the archives on DO, adding O2 and distribution of O2 as well, and I too have a Vertex system. Bottom diffusers aren't necessarily there to add O2, but to distribute it (probably not the right terminology).

The surface aerators do a much better job of adding oxygen than the bottom diffusers. As the warm water holds more oxygen than cold, your Vertex diffusers are evenly distributing the O2 by bringing up the cooler (less DO) water to the surface, mixing it with the warmer (higher levels of DO) so you have a more even amount of DO throughout the BOW, thus eliminating the thermocline (warm on top, cooler on bottom).

My understanding would be that the surface aerator only, would introduce O2 yet it would remain in the upper levels of the BOW.

A combination of both would seem to be the best of both worlds where you're adding O2 with the surface aerator and your diffusers would distribute it evenly.

I'm now in hopes the experts will chime in with a more accurate assessment, and let me know how far off I am.

Keith - Still Lovin Livin

https://youtu.be/o-R41Rfx0k0
(a short video tribute to the PB members we met on our 5 week fishing adventure)

Formerly: 2ac LMB,HSB,BG,HBG,RES