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Green Water Be Gone

There have been several recent postings about how to deal with combating
green water algae.  Here's a method that is easy and cheap.

A little history and then the recipe.  All of my tanks and ponds contain
soil and hardwater and get some window light and/or sunlight.  I don't
add CO2 or plant fertilizers, just feed the fish well and go easy on
tank cleaning.  A couple of years ago I had a green water problem that I
couldn't get rid of by water changes or by adding floating plants.  It
was then that I devised a systematic approach that works for me and
others that have tried it.  It's based on giving the plants several
advantages over algae.  The rationale is explained thoroughly in my book
‘Ecology of the Planted Aquarium’, but here I'm going to give the bottom


First, I do a major (at least 50%) water change to remove the majority
of the algae.  This temporarily reduces the light, pH, and possibly
chemical advantages that green water algae has over the plants.

Second, I reduce iron in the water by adding lots of fresh charcoal to
the filter.  This removes the iron that is naturally chelated to organic
carbon.  (Also, during this treatment time, no chelated iron fertilizers
should be added to the tank.  If the substrate contains soil or
laterite, plants can get their iron from the substrate or they can
temporarily draw on their tissue reserves of stored iron.)  This step is
so critical that if the tank doesn't clear up in 7 days, I'll change the
charcoal.  In the ordinary home aquarium, it’s much easier to starve
algae of iron than it is to starve them of phosphates or nitrogen.

Third, I monitor the pH carefully.  One of the problems with green water
is that their photosynthesis drives the pH up so high that plants have
trouble getting the CO2 they need.  This puts them at a huge
disadvantage with green water algae, which can use bicarbonates much
more effectively than plants.  So if the pH gets up around 8 or so, I
add either vinegar or a commercial product to bring the pH down to 7.5
or less.

Fourth, I reduce the light intensity.  For my tanks near sunny windows I
tape cloth (piece from an old bed sheet) over the back of the tank.
Instead of two fluorescent lights overhead, I use just one.  In choosing
which light to remove, the bulb with the most blue light is the one I
remove, because this is the light that makes iron more available.
During the treatment time, the plants get about 1 watt per gallon of
Cool-white light and some diffuse window light.

Fifth, I run a strip of duct tape along the bottom at the back of the
tank so that the soil layer is never exposed to sunlight.  (Sunlight
plus soil will add a lot of iron to the water.)

Sixth, I add floating plants to the tank and encourage any emergent
growth of rooted plants.

All of these measures give plants small advantages over algae.  While
one measure by itself might not work, when you put them all (or several)
of them together, they work.  Usually, the tank will start to clear in a
few days, but persistent cases should, with possibly a change of
charcoal at 7 days, be history within 2 weeks.  If this method doesn't
work within two weeks, then the plants may not be healthy or numerous
enough to compete with algae.  In that case, the tank has more problems
than just green water algae.

Diana Walstad