Re: Dark algae beneath gravel

 Don Funk <donfu at microsoft_com>, wrote:

>......rarely clean the bottom of the tank with all the plants growing, but
> change 25-30% the water about every 10-12 days which keeps the tank
> very clean. I don't have any problem with algae above the "gravel
> line".  Beneath the gravel there is a dark algae growing which I can
> see looking though the glass on the side where air bubbles also form in
> the gravel...should I worry about this?

The dark algae is probably a mixture of bluegreens (Cyanobacteria).  I see
it all the time, too, growing between the soil or gravel and the glass.  It
is extremely tolearnt of dim light, and I wouldn't worry about it as long
as it stays beneath the gravel.  If it covers your plants, etc., then I
would worry about it.  The bubbles in the gravel could be nitrogen, formed
by denitrifying bacteria using nitrate.  Do you have measurable levels of
nitrate?  I wouldn't worry about the bubbles unless there is other evidence
of a lot of bacterial activity in the gravel (formation of black iron
sulfide, bacterial cloud next to the surface).

Paul Krombholz                  Tougaloo College, Tougaloo, MS  39174