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bluegreen algae control

I've struggled with bluegreen algae (cyanobacteria, cb)  outbreaks on and
off for years now, and I've made a few observations that I'd like to throw
into the mix. In short, I think cyanobacteria and green algae compete in
aquaria.  Cyanobacteria bloom when they get the competitive edge and the
blooms can be controlled and reversed by swinging the competition back to
green algae.

Most of the times I've seen cb problems develop, the cyanobacteria
progress from barely noticeable to tank - dominating over a period of a
couple days to a couple weeks.  Once the cb are established, no amount of
cleaning, water changing or most tank management changes can get rid of
it.  Once it's there, it's in control.

Cyanobacteria outbreaks have ended mostly with chemical treatments.  I've
found that copper sulfate, erythromycin and other antibiotics can wipe out
cb populations.  On one or two other occasions, the outbreaks ended
without major effort.  In those cases, the demise of the cb was
accompanied by accelerated growth of green algae.

I decided that the pattern of cb occurence - blooming from negligible to
dominant without obvious cause, a tendency to stay dominant once
established and sudden disappearances coupled with an increase in green
algae growth - indicated a competitive relationship with green algae.

I tested this idea.  I had a tank with cyanobacteria and no evident green
algae.  The tank had no fish and a moderate population of low-light plants
that mostly seemed uneffected by the cb.  I decided that I would generate
conditions I thought would favor green algae and see what that did to the

I increased the lighting from 1 watt/gallon to 2.5 watts/gallon, removed
the media from the biological filter, started adding a major nutrient
fertilizer with ammonia and continued weekly cleaning and 15% water

The response was nearly immediate, but not spectacular.  The tank
developed cloudy water (phytoplankton) and judging from the amount I
removed at the next cleaning, the cb growth rate dropped to about 1/2 its
previous level.  The cb growth continued to decrease for about a month.
At one month I noticed green algae growth on the tank sides and after the
next tank cleaning, the cb was rare.  Now its two weeks later and the
cyanobacteria aren't visible.

This sounds a lot like the results that DWebb reported a couple months ago
from ammonium sulfate addition, but without the severe buffer depletion
that he reported.

I don't have any reason to believe that higher plants compete with cb the
same way that green algae seems to.  That is, I've never seen that adding
more plants to a tank helps control cyanobacteria.

Any way, now I have some green water that I need to clear up...

Roger Miller
basking in sunny Albuquerque - waiting for the monsoon