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phosphorus and phosphorus control


Recently I spent some time pondering the role of phosphorus in aquariums.

We usually think of phosphorus more in terms of dissolved phosphate, but
(excepting the many biological forms) phosphorus actually occurs in
aquariums in four different forms:  dissolved inorganic phosphorus,
particulate inorganic phosphorus, dissolved organic phosphorus and
particulate organic phosphorus.  The inorganic forms are phosphates and
the organic forms are molecules derived from the decomposition of
biochemicals.  There are important interconversions between the different

What follows is a sad attempt at ascii art that tries to illustrate some
of the more important aspects of the system.

                    Water                 Water
                    changes               changes
                      ^^                    ^^
                      ^^                    ^^
                      ^^                    ^^
                      ^^                    ^^
                      ^^                    ^^
                    ----------           ----------
Water change        Dissolved <<<<<<<<<< Dissolved     
Fish waste >>>>>>>> Inorganic <<<<<<<<<< Organic   <<<<<<<< Fish waste
Additives           Phosphorus           Phosphorus
                    ---------- __        ----------
                      vv       |\
                      vv         \
                      vv          \
                      vv           \
                      vv            \
                      vv             \
                      vv              \
                      vv               \
                      vv                \
                    -----------          -----------
 Fish Food          Particulate          Particulate        Fish Food
 Fish Waste >>>>>>> Inorganic            Organic    <<<<<<< Fish Waste
                    Phosphorus           Phosphorus
                    -----------          -----------
                      vv                   vv
                      vv                   vv
                      vv                   vv
                      vv                   vv 
                    Removal              Removal

The sketch shows each of the four major types of phosphorus, the major
sources for each type, the interconversions between them and the possible
fates of each form.  For simplicity, biological uptake of phosphorus isn't
included in the sketch.  The pathways shown with double lines of symbols
are pathways that might be promoted to reduce problems with dissolved
phosphorus levels in an aquarium.  The pathways shown with single symbols
are pathways that might be supressed to reduce phosphorus problems.

This whole thing could be given a lot of words, but I'm going to try
to limit myself to a brief description.

*Dissolved inorganic phoshorus* is biologically available and essential to
plants and algae.  It is mostly dissolved phosphates.  It enters the
aquarium from tap water, sometimes from water treatments like pH buffers
and as inorganic phosphorus wasted through the gills and kidneys of
animals.  It is removed from the aquarium by water changes.  It is
produced in aquariums from organic phosphorus by bacterial and algal
phosphatase activity.  It is converted to particulate inorganic phosphorus
by sorption and precipitation.

*Particulate inorganic phoshorus* is mostly not biologically available. It
is phosphate associated with phosphate minerals and adsorbed on metal
hydroxides and other solids in the aquarium.  It enters the aquarium
mostly in fish food and animal feces and it can be removed by siphoning
out detritus and cleaning filter media. It is formed within an aquarium by
sorption and precipitation of dissolved inorganic phosphorus.

*Dissolved organic phosphorus* is biologically available to bacteria and
possibly to algae, but I haven't found references saying it is available
to plants.  It enters the aquarium from animal waste. It can be removed by
water changes.  Bacterial and algal phosphatase activity convert dissolved
organic phosphorus to phosphates.

*Particulate organic phosphorus* isn't available to plants and algae, but
is available to animals.  It enters the aquarium as plant detritus, fish
food and feces.  It can be removed by siphoning and filter cleaning.  It
is converted to dissolved phosphates by phosphatase activity.  The
phosphatase activity is partly due to detritivores but also to bacteria
and possibly algae.

I wrote a little spreadsheet program to balance the reactions and provide
estimates for steady-state concentrations of each component with various
estimates of the reaction rates.  If there is interest then in later
letters I can try to summarize what that all seems to mean to controlling
phosphorus levels in an aquarium.

Roger Miller