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I have an observation to share.  Several of us have reported
cyanobacteria ("CB" -- the stinky blue-green slime/sheet algae which is
really bacteria) between the gravel and front glass of our aquariums.
This layer can appear in otherwise very well run aquariums.  If memory
serves, George Booth even mentioned using a special tool to pluck the
nasty stuff from between the front glass and gravel in his Dupla-style
aquariums with heating cables.  This layer of CB recently appeared in an
otherwise very successful 125 gallon tank.  Interestingly, in the fall
the late afternoon sun strikes the right side of this tank for a couple
of hours.  The CB appeared at gravel level just where the sun strikes
the tank (perhaps suggesting that strong light is a positive factor for

Use of erythromycin has been suggested for treating CB on a temporary
basis.   I also believe someone suggested that CB may thrive in a
nitrate-limited environment.  My nitrate levels are often at or near 0
ppm.  I did two things to treat the CB:  I added nitrates (KN03) to
raise the level to around 5 ppm and I chopped up one erythromycin tablet
into 4 pieces and inserted them in the gravel spaced about 3 inches
apart where the CB was growing at the front glass.  No other tank
parameters were changed.  I continued to do 40% water changes weekly.
10 days later all signs of the CB are gone.  It took several days for
the erythromycin chips to dissolve and several additional days beyond
that for the CB to disappear. I can't say whether the additional
nitrates helped or not, but if another outbreak occurs I promise to try
one and then the other solution, and not both at the same time.

What I like about this solution is that one erythromycin tablet is
supposed to treat 10 gallons of water for whatever fish diseases it is
used for.  At one tablet per say, 110 gallons of water placed in the
substrate the concentration of antibiotics must have been very low.  The
plants in this area, Glossostigma and E. tennellus, showed no signs of
distress during this treatment.  

Regards, Steve Dixon in San Francisco