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phosphorus and phosphorus control (3)
Water change Dissolved <<<<<<<<<< Dissolved
Fish waste >>>>>>>> Inorganic <<<<<<<<<< Organic <<<<<<<< Fish waste
Additives Phosphorus Phosphorus
---------- __ ----------
Fish Food Particulate Particulate Fish Food
Fish Waste >>>>>>> Inorganic Organic <<<<<<< Fish Waste
Two days ago I described the forms of phosphorus shown in the diagram and
listed the mechanisms that add and remove each form. Yesterday I
described the major routes in and out of the aquarium and some means of
using those routes to control the phosphorus levels. Today I guess I'll
write about the reactions within the aquarium and how they might be used
to help control the concentration of dissolved forms of phosphorus.
The terms for different forms of phosphorus are repeated frequently in
this discussion. I'm going to use some abbreviations to keep the text from
becoming far too cumbersome:
DPi is dissolved inorganic phosphorus
PPi is particulate inorganic phosphorus
DPo is dissolved organic phosphorus
PPo is particulate organic phosphorus
*Phosphatase* is an enzyme that bacteria use to convert DPo and PPo to
DPi. Some algae may also have that ability. Alkaline phosphatase - which
works when the pH is near and above 7 - is the most well documented form
of phosphatase. There is also an acid phosphatase that doesn't seem to be
as well documented. Phosphatase activity increases with the size of the
bacterial population; activity is promoted by oxygen and suppressed by
Bacteria typically produce more phosphate than they consume, and that can
lead to an excess of phosphate in the water.
The phosphatase-catalyzed reaction from PPo to DPi should be discouraged
to prevent production of excess dissolved phosphate. If the reaction can
be slowed down then phosphorus will remain in the detritus where it can be
more easily siphoned off before it causes problems.
The reaction from DPo to DPi is a somewhat different matter. My reading
indicates that some algae may be able to utilize DPo but plants cannot.
If that's true then DPi - which can be utilized by plants and algae - are
preferred to DPo, so the conversion of DPo to DPi should be encouraged.
How do we suppress the phosphatase-catalysed reaction from PPo to DPi
while we still allow the phosphatase catalyzed reaction from DPo to DPi?
I don't know, but algal scrubbers come to mind. My impression is that
this is probably a fairly fast conversion that would be difficult to
I can speculate about how we might reduce the rate of
phosphatase-catalyzed reactions, but I don't have any real data. The
conditions that promote high levels of phosphatase activity and so cause
rapid release of phosphorus from detritus are exactly the conditions found
in aquarium filters. In a filter, organic particulates are trapped in a
constant flow of aerated water where a large population of bacteria can
act on the detritus. The flowing water also flushes the phosphate
released by the reaction, which otherwise could suppress further
phosphatase activity. So removing a filter should reduce the phosphatase
I keep unfiltered tanks. I think in retrospect that pulling the filters
off my tanks helped control phosphate levels, but I can't substantiate
that. Has anyone measured phosphates over a period of time after removing
the filter from a planted tank? Or observed changes in a tank after
removing the filter that would suggest a change in phosphate levels?
*Precipitation and sorption* convert DPi to biologically unavailable PPi.
This is a group of reactions that in nature are largely responsible for
keeping phosphorus in a growth-limiting role.
DPi combines with calcium to precipitate a number of insoluble solids.
Dissolved calcium (hardness) and pH near and above 7 promote formation of
the solids. The solids don't precipitate readily and may not form at all
without a suitable substrate or "seed" to grow on. Industrial and
municipal-scale phosphorus removal systems use several different kinds of
seeds. One system uses plain sand as a seed for forming the
phosphorus-bearing solids, other processes use calcium carbonate (which
isn't stable under common conditions in planted tanks) or one of several
kinds of calcium phosphates. Dissolved organic compounds like tannins can
foul the seed surfaces and slow down the precipitation of calcium
Very soft water and water with a low pH should be prone to elevated
phosphate levels because the DPi won't precipitate. If you have excess
phosphate in a tank with soft water or pH below 7 then you might be able
to increase the calcium content and pH of your water and add a fresh seed
material to your filter media to promote precipitation of calcium
Dissolved phosphates also have a strong tendency to stick to many kinds of
surfaces. The generic term for this process is sorption. Phosphorus is
very stongly associated with surfaces on ferric oxyhydroxides and aluminum
hydroxides. Ferric oxyhydroxides are common in nature. In most soils -
including aquatic soils - it is often sorption on ferric compounds that
keeps phosphorus in place and out of the water.
Low levels of dissolved phosphorus can be removed from water by dosing
with a ferrous chloride solution. The ferrous iron oxidizes to ferric
iron and precipitates as the oxyhydroxides. DPi is strongly associated
with the iron and settles out with it. That process also lowers pH and
alkalinity. DPi may also attach to laterite and other soil substrates
that contain ferric oxyhydroxides.
Several companies sell filter materials on the aquarium market that use
the same mechanism to pull phosphate out of the water. I think that those
products use activated aluminum hydroxide. In my experience, the products
work, but often not for long and they can be fairly expensive to use on a
regular basis. They attach a lot of compounds other than phosphate.
Those other compounds (particularly silica and some organics) can pretty
quickly foul the material and ruin it's capacity to remove phosphate. I
suspect that phosphate-adsorbing products would be more effective when
used as a pre-treatment to remove phosphorus from tap water than they are
as an aquarium filter medium.
In my last letter on this topic I'll try to summarize all this and use the
results of a simple spreadsheet model to assess the relative effectiveness
of different control methods.