Edit: bacteria and algae will grow as the tank matures .. they will generate nitrogen/oxygen bubbles .. doesn't matter whether they are under or on top of the sand.
Thanks guys for the help. The bubbles are on top of the sand on a corner of the tank which doesnt have the greatest flow there. The tank has been cycling for 2 months now. The sand where the bubbles are present is a browish red colour.
Erbio you got a 540G tank..wow.. do you have any pics of it?
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