It takes some pretty exceptional circumstances for there to be a lack of oxygen. Oxygen will always be trying to come back into the tank via the surface, and if the surface is disturbed in any way, like with a filter output, the oxygen transport will be even more rapid.
I can think of only three cases in a typical aquarium where oxygen deficiency would be present. The first would be in a heavily stocked tank that is also heavily planted with the owner using CO2 injection. Because surface disturbances are minimized to keep as much CO2 as possible in there, and when the lights are out the plants take in oxygen instead of releasing it, then the water can become devoid of oxygen. The only other one is when medications are being used. A fair amount of medication works by oxidizing organic material, and it uses the oxygen right out of the water. The third would be if you are keeping a lot of cool-water species in too warm of a tank. Cooler liquid hold more dissolved gas, so fish from cooler waters will be used to and need water with more dissolved oxygen than warm water can even hold.
However, if none of these situations apply to your situation, then it is exceptionally unlikely that oxygen deficiency can occur. When there is a lack of oxygen in the water, the system is out of equilibrium, and nature will always tend toward equilibrium. In this case, equilibrium is well-oxygenated water, so oxygen will always be making its way into your water.
/www.tetra-fish.com/catalog/product.aspx?id=243) but I am not comfortable with the results. Are there any other oxygen test kits out there?
