The best way to test the amount of dissolved oxygen in your aquarium's water is with a portable dissolved oxygen meter. After calibrating the meter, you can place a probe into the tank water, and read the results on the digital display. Meters usually express the results as a percentage of dissolved oxygen or by determining how many milligrams or oxygen are in each liter of water.
Chemical Test Kits
Unfortunately, dissolved oxygen meters are expensive and outside the budget of most casual aquarists. The alternative is to use a dissolved oxygen chemical test kit that works similarly to other chemical test kits, such as those that measure the pH, nitrites or nitrates. Research testing kits before purchasing them, as some are known to produce flawed results when used in conjunction with other chemicals and water treatments.
If your aquarium water does not contain enough dissolved oxygen, you may notice your fish behaving differently. For example, bottom-dwelling fish must often swim higher in the water column when oxygen levels are low. You may also see your fish gasping for air at the surface or spending their time near the filter output, where oxygen levels are at their highest.