OK, just googled disolved oxygen and it says that the level is affected by the level in the surrounding air (at 5K I would suspect your oxygen content is lower than mine here about 200 feet above sea level), aireation and what the plant gives off. I would say yu definately want to compensate by running a lot of air stones in you res.
Here's what I use:
This pump:
http://www.discount-hydro.com/productdisp.php?pid=412&navid=8
With there airstones: I use the 4"
http://www.discount-hydro.com/productdisp.php?pid=548&navid=8
The pump comes with a 4 way spliter and I use 2-3 per 10 gallon res and the water just boils and spalshes which is what I want! Big air also soves or prevents lots of root problems.
I'd also see what I could do about getting the humidity up over 60% at least for veg and then lower it to under 45% during flower. It will help the plants breath.