John H. wrote:

Hi, I was doing a water change for my 60 gal planted tank (florite/gravel
mix) and while I was cleaning the gravel, noticed
a lot of bubbles being liberated... is this CO2 ? O2 and shoud I be worried
about it?

It could be either O2 or hydrogen sulphide (H2S). H2S is deadly. If you
noticed a rotten egg smell when the gas was released, then this was H2S.
I have large O2 bubbles created by the roots of my plants. These
periodically force their way up through the gravel to the water surface.
This is a good sign. On the other hand, areas of gravel that remain
undisturbed and are not aerated for long periods of time promote
bacteria that become anaerobic. These areas in my tank become black. I
can only see these when they are up against the glass. When they are
finally disturbed - gravel cleaner, fish digging etc. - they release
bubbles that smell like rotten eggs. I have heard that if fish get a big
enough waft of H2S it can can kill.

I think this is more of a problem with sand substrates, where the sand
can get compacted. Allowing plants to grow completely across the
substrate should help prevent that as the roots produce O2. Maybe you
simply have O2 bubbles being produced by the roots?

Nikki