bubbles of death under my gravel?

[ph7]

See attached (awful quality camera phone) photo. Below the soily substrate in this old planted aquarium, are these the gas-of-death bubbles that "shall not be named"?

Is this a death trap? A ticking time bomb?

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[jsoong]

Possibly, but unlikely in a planted aquarium. The plant roots aerate the soil so anaerobic pockets don't develop. Chances are, bubbles in the soil are nitrogen (bacterial denitrogenation - changing nitrates back to N2). You can stir a bit up & if it has no smell, it is prob. OK.