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Beating back bluegreen algae


Several years ago I read an interesting text called "Environmental
Microbiology".  I think it was there that I found the statement that blue
green algae are essentially ubiquitous in nature but rarely dominate the
environments where they live.  The cases where bluegreen algae do dominate
are those where conditions are so extreme that green algae and plants
cannot thrive; e.g. high nutrient levels, low nutrient levels, high
temperatures, low temperatures and so on.

On that basis it's always seemed to me that when bluegreen algae comes to
dominate a planted aquarium there must be something extreme about the
conditions in the tank that are unfavorable to the growth of plants and
green algae.  The natural extension to that thought is that bluegreen
algae in an aquarium could be controlled or eliminated by promoting the
more moderate conditions that favor plants and green algae.

About this time last year one of my 10 gallon tanks developed an
infestation of bluegreen algae.  The tank is lit with 2 15-watt
flourescent tubes (3 watts/gallon) and choked with Vallisneria.  It also
housed some C. wendtii and an algae-covered bit of driftwood that I'm
rather fond of, a few male common guppies, an oto, some ghost shrimp and a
female American flag fish. The tank was unfiltered but circulation was
provided by a small powerhead.  I fed very little to encourage the fish to
eat algae.  The bluegreen scourge started out slowly but after a few
months I was cleaning off the driftwood and the Val every week and
siphoning the bluegreen slime off the bottom.  I had no particular plans
for the tank so I let things go along like that.

A few weeks ago I decided to use the tank to grow out some Barclaya
longifolia seedlings, so I needed the bluegreen algae out of there.  
Instead of nuking the tank with antibiotics I decided to try a more
natural cure.

I started changing conditions in the tank not so much to discourage the
bluegreen algae as to encourage the growth of green algae and plants.  
The changes took several weeks.  First I thinned out the Val and planted
some E. tenellus in the tank (variety is good).  Then I started feeding
the fish more heavily to increase the nitrogen and phosphorus supply.  
Finally I started using a commercial aquatic plant fertilizer and added
yeast-generated CO2 from a 1-quart generator.

The growth rate of the bluegreen algae declined only slightly and things
remained pretty well unchanged until I started the CO2, then the pieces
appeared to fall together abruptly.  The bluegreen algae didn't disappear
suddenly but it did cease to grow and start disappearing.  I gave the tank
a fairly thorough going over last weekend when I changed water and removed
what I could.  What was left after the cleaning is nearly gone now.  The
tank is almost complete free of bluegreen algae.

I've tried similar cures a couple times before.  I've sometimes even gone
to the extent of triggering a green water bloom.  As Dave Whittaker
pointed out recently, green water combats many other types of algae.  
That seems to include bluegreen algae.

The method is pretty simple.  Don't try to kill the bluegreen algae.
Instead, combat it by encouraging the growth of competing plants and green
algaes.  Identify extreme conditions and fix them - give your plants the
best chance to fend for themselves.

Roger Miller