Bubbles on top of my tank

Question
Why are there bubbles on top of my tank. we're using an under gravel filter.

Answer
Hi Ryan

Could be from any chemicals you've added, like some water dechlorinators will do that.  Is there a male betta in the tank?  They blow bubble nests.  I'm not real familiar with under gravel filters-too much of a hassle for me lol, is there any surface agitation?  Or is the filter output just below the water surface?  

Christy