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phosphorus and phosphorus control (2)


Well, I guess there was enough interest after the first letter to justify
my going on.  Too bad it got this long...

First to David Youngker's comment about my purpose...

My goal is to help people find ways to control excessive algae growth
caused by high phosphorus levels.  I think it should be possible to
control those problems with good tank-management habits -- without large
investments in chemical test kits, special filter media or a minor in

And on to other things...

Yesterday I foisted this diagram on the list:

                    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 most evident controls on all the phosphorus types is to reduce the
phosphorus sources (fish food, fish wastes, and phosphates in tap water)
and increase removal of phosphorus with larger or more frequent water
changes and more effective removal of phosphorus-containing sediments.

*Water changes* seem like a great way to keep dissolved forms of
phosphorus from building up in a tank, but that may not be as effective as
it seems at first blush.  In my case, for instance, I do 15% water changes
in every tank, every week.  Along with 15% of the water, the water changes
remove only 15% of the phosphorus. Commonly recommended change schedules
like 20-25% every two weeks (equivalent to 10% to 12.5% per week) are even
less effective.  Still, 10% is better than nothing and for most of us
practical limits on time and energy keep our water changes at this sort of
low level.

People who have significant phosphate in their water supplies have a
different problem.  For them, water changes may actually increase rather
than decrease the phosphorus supply in their aquarium.  For those people
smaller water changes may be better.  They may need to use water run
through a commercial phosphate-removing media like activated alumina, or
purified (ultrafiltered or deionized) water.

*Sediment removal* - usually done just by siphoning off detritus and
cleaning filters - is another great-sounding ploy, but here too there are
some snags.  In a bare tank it's easy to remove all the visible detritus
during the periodic cleanings and to wash all of the built up material
from filters.  It's a lot harder to get the detritus out of a heavily
planted tank, but even with heavy planting it should be possible to remove
more than 50% of the detritus at each cleaning.

Most of us clean our tanks and filters on a periodic basis.  Between
cleanings the accumulated organic particulates can be attacked by bacteria
and release dissolved phosphorus.  By the time we remove the accumulated
particulates we may be getting only a small fraction of the phosphorus
that it originally contained.

On general principles filters should be cleaned as frequently as possible
to limit the release of dissolved phosphorus.  Unfortunately most of the
popular aquarium filters are difficult enough to get into that daily
cleanings are out of the question.  For most of us, daily siphoning of
detritus is also out of the question.

Both cleaning and water changes provide important controls on the
accumulation of phosphorus in the aquarium but they are of limited
effectiveness.  There are a few additional options for phoshorus removal
that depend on conversions within the aquarium and I'll describe those
when I get around to writing about those conversions.

That brings us to the options for controlling the additions of phosphorus
to our aquariums.  It's pretty easy to avoid adding phosphate fertilizers
or other phosphorus-containing chemicals to the tank.  It's more
difficult, but possible, to avoid adding phosphorus to the tank in the
water supply that's used for water changes.  Once those inputs are
controlled, we're still left with a large influx of phosphorus.  That
influx is from feeding, either directly as uneaten food or indirectly as

*Feeding* can be controlled in two different ways; in quantity and in
quality.  Controlling the quantity of feedings is pretty straightforward.  
Many of us (including myself) probably feed more than is necessary for
maintaining healthy adult fish.  Since feeding is the major source of
phosphorus for the tank, reducing the feeding can have a proportionate
effect on the phosphorus level in the tank.  Decreasing the feeding rate
by 30% can reduce the phosphate level in the tank by 30%, a 50% decrease
in feeding can halve the phosphate levels, and so on.  Most of us are
probably pretty happy with the way we're feeding our fish and may not be
comfortable with large reductions, so there's a limit to the control that
can be attained this way.  Probably the most comfortable way to reduce
feedings is to reduce the fish population in a planted tank.  That lets
you reduce the feeding without running the risk of stressing your favorite

Controlling the quality of the feed is not as straightforward.  The main
idea is to use food with a lower phosphorus content.  Many prepared foods
contain more phosphorus than fish need and not all (perhaps less than
half) of the phosphorus is even in a form the fish can use.  Look for
foods with lower phosphorus content that are formulated for the specific
needs of your fish.  You can even do this with live and fresh foods if you
can find the phosphorus content of the foods you're using.

There is also an approach for those who want to force phosphorus to be
more growth-limiting than nitrogen.  Feed your fish a diet with a ratio of
at least 7:1 between available nitrogen content and available phosphorus
content.  At that ratio, phosphorus should be in short supply compared to

It's fairly easy to check this with prepared foods.  The total nitrogen
content can be estimated by dividing the crude protein content by 6.25.  
Probably most of the nitrogen content will be biologically available.  
Total phosphorus content should be listed on the packaging.  As little as
half the phosphorus content may be biologically available, but it's safer
to assume (at least initially) that all of it will be biologically
available.  If you use a diet that is 45% protein (for instance), then it
will contain 45/6.25=7.2% nitrogen.  In order for phosphorus to be
growth-limiting the phosphorus content of the food should be about 1/7th
that amount, or 1% of the total.  If the actual phosphorus content of the
diet is 1% or less, then phosphorus should be growth limiting.  This is
not as easy when using live or fresh foods but as long as you can find the
protein and phosphorus contents of the food you can use the method.


Hopefully that answered some of the initial questions left after my first
letter.  In another letter I'll try to describe some of the
interconversions between forms of phosphorus in an aquarium.  I'll even
give thought to how they might be used to control phosphate levels in an

Roger Miller