The bubbles are created by the venturi, an additional feature of the filter.
The lower the unit sits in the water, the harder it is for it to pull air into the unit, which will reduce the presence of the bubbles. It is also important that the venturi isn't blocked. The circulation of water through the filter will naturally introduce oxygen into the water.
If you wish to increase the oxygen level, you can also direct the flow deflector towards the surface of the water.
In order for the venturi to work, the venturi assembly must be slightly above the water level. The venturi is not essential for operating a healthy aquarium.
If it stops working this normally means it is time to clean the filter, the venturi needs good water flow in order to provide the correct level of pressure, when foams become blocked the flow reduces.
If there is any grease on the glass, this can stop the suckers from adhering initially.
Often, in a new aquarium, slime will build up initially, which also affects the suckers.
We recommend cleaning the glass and then run hot water on the suckers before pressing firmly on the glass area, where you wish to have your filter.
These are included with the replacement impellor.
If you are using the venturi, then there will be some noise from the air being drawn in and forced into the water flow under pressure. You can turn the venturi down.
However, we would also recommend that the following checks are carried out: