The first thing I would do is by a R/O. To me it is just as important as a skimmer, wet/dry, pump, LS, LR, etc.......
When it comes to the increase of DO (dissolved oxygen) in an aquarium, the water surface is one of THE main places where this exchange takes place. Oxygen is dissolved down into the water, carbon dioxide is released out into the air, not to mention that other gases readily pass through the permeable surface of water as well. This is the principle that our bodies work on when we breathe; good air in... bad air out. It's the same way with your tank.
Dissolved oxygen levels in an aquarium can be affected by a number of factors. The greater the temperature, the lower the dissolved oxygen. The greater the salinity, the lower the dissolved oxygen. The more fish in the system, the lower the dissolved oxygen.