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Actinic lights in freshwater plant tanks and algae problems

Diana Walstad's book "Ecology of the Planted Aquarium" certainly is full of
information that I've never heard before....

Frequently, people post about persistent problems with algae in their tanks,
and quite often their list of "tank parameters" include high color
temperature and often actinic lighting.

In Chapter X of her book, on "Algae Control", Diana discusses Iron as the
limiting nutrient for algae. In oxygenated water, iron is generally found
bound, either as an iron precipitate (FeOOH, FeCO3, etc.) or is is bound to
dissolved organic carbon (DOC). Free iron (Fe++, Fe+++) are the only types
of iron that algae can use. So far, so good.... nothing really new here...

Diana goes on to explain that while rooted plants can get iron from the
substrate, algae depends upon free iron in the water column. Again, nothing

But she explains that a common process called "photoreduction of iron" is
able to set the bound iron free:

DOC-Fe+++  +  light -> Fe++  +  oxidized DOC

Apparently, this reaction, which also takes place with manganese and copper,
can be greatly accellerated by light.

Now this is "slightly new", at least to me... but the kicker is yet to

Diana cites research which indicated that UV and blue light induce the most
photoreduction because wavelengths of light below around 500 nm are
energetic enough to break the DOC-Fe+++ chemical bonds. She also explains
that iron can bind to a variety of chemicals and different types of DOC, and
that each separate "species" can vary in susceptibility to photoreduction.

Now, to me at least, this IS new information, and another reason to stay
away from 50/50 and actinic lights in freshwater plant tanks, especially
those with older substrates and corresponding high levels of DOC in the
water column.

Many people (most of us I guess) use certain "aim points" for particular
nutrients in the water column - we want iron, to be at one level, phosphate
below another level, etc. Many of these "aim points" come from hydroponic
studies. Diana reminds her readers that in tanks with enriched substrates
(her's are soil based), iron should really come from the substrate, not the
water - so our additions of chleated iron to the water column might be
counter productive and based upon research which is not really applicable or
appropriate for an aquarium.

Comments or thoughts, anyone???

James Purchase