There is a thread by BigToke about bio buckets that touches on o2 levels in the water. He uses a dissolved oxygen meter and has tested multiple ways of dissolving oxygen. He claims that a waterfall (nutes returning to the rez) of 8-12 inches is the most effective method. Its so effective that he doesnt even mess with ph or even change out the rez. Look the thread up if you want a good read. Also hit up cravinmorehead if you have questions regarding the biobucket system as BigToke is no longer around. The only problem with the waterfall is the noise.
Where I live, even though I grow indoors, mosquitoes get into the house. Open water systems are a magnet for them, so I have to use closed systems. My max is 4 gallons of nutes