Dissolved oxygen can easily be manipulated with either air pumps and air stones, aiming return nozzles at the surface of the water, or a water pump that vertically pushes the water to the surface to allow for gas exchange. In a typical reef tank most O2 is lost and transferred with CO2 at the surface of the water. Another large sink for dissolved oxygen to be consumed is settled detritus, food, and other dissolved organic compounds. As the process breaks down organic compounds dissolved oxygen will be consumed. For over stocked tanks that can cause a huge issue as your animals will consume the oxygen as well as their waste which could lead to dangerously low levels of dissolved oxygen.