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Cyanobacteria observation and question

I went skiing last week and noted that when I came back, the cyanobacteria
in some areas of my tank had begun to spread again.  This led to an
observation.

I had been adding daily dosages of Flourish at the time the cyanobacteria
outbreak occurred.  I suspect that if my Flourish additions affected the
cyanobacteria, I would've never had an outbreak.

My dosages of powdered limestone in my tank are added on a weekly basis, so
effectively, I never missed a dose of calcium carbonate.  This would lead
me to believe that the calcium carbonate is not a major factor at least in
this stage of the game.

My CO2 generator has begun to run low, so this might be a factor.

I've been adding roughly 1/8 tsp of ammonium sulphate (maybe a little less)
to my tank daily for the last few weeks.  Prior to adding any, I made some
measurements to determine how much I would be raising my ammonium
concentration on a daily basis, and calculated it to be less than .25 ppm.
I haven't noted any ammonia accumulation in my tank over the last month.  

When I returned from my ski trip, my snails were not doing much towards
eating the cyanobacteria, allowing it to spread again, and possibly causing
some of the babies to starve, potentially releasing additional
phosphate-heavy nutrients into the water.  I've resumed dosages and am
waiting to see if the trend will reverse again.

Here's my question:  Does the term "cyanobacteria" refer to the bluish
color that we see in our aquariums, or does it mean something else?  Many
of us believe that cyanobacteria are capable of fixing atmospheric
nitrogen.  I haven't seen experimental evidence that this is true, but it
sure looks that way from my experiences with my tank.  I'm wondering if
cyanobacteria may produce internal toxins to make it taste bad or even make
it poisonous, but if these toxins might be a by-product of nitrogen
fixation (CN- anions, perhaps).  I'm further wondering if the presence of
an easier to metabolize nitrogen source such as NH4+ or NO3- might reduce
the ability to produce such a toxin, if it exists.

This is a lot of reaching and speculation, so I hope that some of the
medical/botanical/microbiological types here can shed some light on my
question.

David W. Webb           Corporate Business Systems
Texas Instruments Inc.  Dallas, TX USA
(972) 575-3443 (voice)  MSGID:       DAWB
(972) 575-4853 (fax)    Internet:    dwebb at ti_com
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