Hi naikoob,
The bubbles forming at the surface of your tank water are probably bubbles emitted from plants when the oxygen level of your tank water reaches a saturation point. It is more evident when a layer of scum is formed due to protein build up from fish food.
On the side note, you might want to test for presence of ammonia (the bubbles could also be ammonia gas which is deadly to fishes if your pH is above 7) as your tank could well be in the cycling phase.
Plant Physiology by Taiz and Zeiger
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