Many things can affect the dissolved oxygen in your aquarium. The most important things to remember are stocking levels and cleaning. Too many fish will require more oxygen than is available in the water. Fish waste, decaying food and rotting plants are all consumed by bacteria, which also require oxygen. Regular cleaning, water changes, filter maintenance and proper feeding will ensure that the bacterial demand on dissolved oxygen is under control.
There are also many things that you can do to improve dissolved oxygen in your aquarium. Most of the dissolved oxygen in your aquarium comes from the atmosphere; this is accomplished through interaction with the water surface. Increased water surface area gives your aquarium higher potential for atmospheric interaction. Turbulence and water movement along with good aquarium design will increase your aquariums water surface area, and dissolved oxygen. Use of an air pump and air stones or a power head with a venturi aeration feature will also greatly increase your dissolved oxygen. The tiny bubbles created have huge amounts of surface area and greatly increase the potential for oxygen to dissolve into the water. Temperature also plays a significant role in dissolved oxygen levels. Concentration of dissolved gas decreases with increase in temperature, so avoiding high temperatures (above 85 Fahrenheit) in very important. Taking all these factors into account when setting up an aquarium will go a long way to ensuring your long term success.