Part of a FAQ on Aquarium Systems website.
Why does my SeaClone™ protein skimmer periodically put so many fine bubbles into the tank?
During the first few days of operation the skimmer has a tendency to put a small amount of bubbles into the tank; this is normal and the bubbles should dissipate during the skimmer's break-in period (2-4 days). It has also been observed on many occasions that the addition of products which claim to promote a thickened slime coating on fish tends to make the skimmers put bubbles into the aquarium for a wek or so. This problem usually dissipates over the course of one or two weeks.
If bubbles persist, however, it is usually an indication of too much water flow and not enough airflow. You can check this by looking at the depth of submersion of the venturi air intake (located at the Maxi-Jet™ pump intake). The venturi should be 2 to 2.5 inches below the water surface. Often, the venturi ends up about 4 inches below the surface so it may be necessary to cut off a short length of the "J" tube to raise the pump to the appropriate level. Raising the pump closer to the water surface will increase the airflow and decrease the water flow.
Once the pump is set at the proper level remove the cap from the air intake adjustment bulb. This will be the setting which allows for maximum air flow and minimum water flow; there should be no bubbles going into the tank at this setting.
