Sometimes a layer of bubbles is on top?

i have an aquarium with 2 tetras fish in it, that has an under gravel filter system in it that uses a water Stone for water cycle and aeration., every now and then their would be a layer of tiny bubbles on top of the aquarium, i bought it last year december. wondering if the bubbles are normal and if they will go away eventually? oh yeah i use tetra aqua aqua safe water conditioner that has bio extract in it.