Stopping the bubbles?

[csmaster]

My tank has an internal filter which is fully submerged, but every few minutes it will spew out a bunch of small bubbles, I dont think there is any air traped in it as its been doing it for about 2 weeks, the only thing I can think of is the bubbles off the plants are getting sucked in, but there arent that many bubbles. does anyone know how to stop these bubbles?

[FishBait]

I'm assuming there is a sponge prefilter on this? If so, when you remove it from the water for cleaning it could be trapping a good amount of air in it. Can you see alot of bubbles on it? Maybe you just have some algae growth on there producing tiny bubbles that accumulate over time?