Although many internal filter units contain aeration adaptors, your filter does not have to make bubbles. Unlike most external filters, internal filters will often not have the cascading effect that brings additional aeration and oxygen to your aquarium. However, most systems will have oxygen incorporated at the surface of the air-water interface. If your aquarium is not packed with many large fish (or species like goldfish, which require 20 gallons per fish), you will not likely need additional aeration.  If you are unable to test your tank's oxygen levels, clinical signs of low dissolved oxygen are fish laying on the bottom and exhibiting increased respiratory effort. You can always add a secondary aerator if you are concerned about the oxygen levels in your tank. It is almost impossible to add too much oxygen since the excess will bubble off at the surface.