Ya know, it doesn't come with it, or mention adding one in the manual. I guess that theoretically, the mister running 24/7 would oxygenate it, but I added a stone as soon as the roots hit the water.
It is my belief that with HID lighting and co2 enrichment, that only leaves o2 to the roots as "the bottleneck" (so long as environment and nutrients are managed properly), so I figured the more air, the better. I opted for the "boss hog 2x diffuser" on a commercial pond pump. It keeps the reservoir foamy and constantly churning. It's also nice that when I add ph adjust or nutes, it mixes itself.
I'd had some setbacks early on, but now growth is superb! I just wish that I did have a secondary res, because about 10 gallons in the res with 6 fast growing mosters is kind of hard to keep stable. I'm constantly chasing ppm and ph.