What is causing air bubbles?

I keep getting air bubbles built up along the front of my tank in the sand bed. What is causing this?

 

I notice a lot of bubbles when my lights have been on a while, and in my case it is oxygen being released by algae. It could also be nitrogen release from a completed nitrate cycle, thanks to the critters that live in the sand bed.

 

This is most likely nitrogen gas from the nitrifying bacteria in your sand. I have it too. Ever now and then I catch a bubble going up the water column from the sand somewhere. I think they're throughout the bed.

It must be a good thing!

 

my clown fish (Coral) "burps" up air bubbles. Is this a bad thing?

 

I would like to bump this thread for a better answer from someone if possible. I to just saw I had the same exact issue(bubbles in my sandbed) and I would like to know if this is a good thing or a bad thing???

TIA