Air bubbles in the BC tanks are usually caused by not having enough water in the back chambers. Try adding some more water.
If you don't mind tanking a knife to your tank....
Some people upgrade the pump to a Maxi-Jet 900, replace the bio-balls with LR
rubble, remove the carbon filter cartridge, and then modify the plastic wall between chambers 1 & 2 to allow more water to flow through. Water is then added to increase the levels in the chambers to within 1" of the top.
A small clown would work with the goby.