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Re: phosphorus andn phosphorus control

On Tue, 23 Nov 1999, Elliot Williams wrote:

> So what foods are low in phosphorus but not too low?

There are actually two issues here.  The first is phosphorus content
and the second is phosphorus digestibility.


I did a quick check about my house and found that Tetra was the only make
of fishfood in the house (I found 3) that listed phosphorus content.  Of
their several products, the new TetraMin Pro line contained higher
protein, lower minimum phosphorus and higher fat content then their other
products. Still not low in phosphorus, but lower.

I suspect that phosphorus content for other brands of prepared fish foods
can be obtained from the manufacturer even if it isn't posted on the
container.  Phosphorus in other forms of food (including live and fresh
foods) are probably available either from the supplier or from published

In salmonid culture high fat content is important for controlling nutrient
pollution from feeding growing stock.  The principle is very simple; fish
have a minimum calorie intake required for growth and if they can't get if
from fats then they burn proteins and other calorie sources for energy and
more of the food's nutrient content is wasted to the water.  I suspect
this principle will work in grow-out aquariums too.  It probably won't
effect aquariums with a mature, non-breeding fish population.

I'd like to find some information on phosphorus requirements for fish.  I
know it's out there.  I'm pretty sure that the total phosphorus content in
most foods far exceeds the fish's requirements, partly because of the
digestibility issue below.


Foods used in aquaculture can be rated for the percent of total phosphorus
(or any other nutrient) content that can be used by the fish.  In those
cases, the foods are designed for and fed to a single species of fish and
things are relatively simple.  In aquariums we deal with mixed populations
of fish and generally we support some scavengers and detritivores, so the
situation isn't as simple.

The largest part of most prepared foods is fish meal.  The fish meal has a
high phosphorus content but a large part of that is in the ash fraction of
the food.  It consists of ground up bone and scales that aren't digestible
to most fish.  Yet another part of the total phosphorus content is part of
the vegetable matter and it is undigestible to most fish.

The undigestible portion of the food ends up in the feces and becomes part
of the particulate fraction.  If a tank is regularly cleaned then at least
some of the undigestible fraction will be removed before it becomes
available to plants and algae.

The digestible part can be used for growth or egg production.  But in
mature non-breeding fish it can only be used for maintenance.  In
maintenance I think that any phosphorus that the fish use will be balanced
in the waste stream by phosphorus that they eliminate through their gills
and kidneys in dissolved organic or inorganic forms.  Those forms are
immediately available to plants and algae.  They're unavoidable if
you're maintaining fish, but it would be good to minimize the input.


So from all this, there appear to be a couple things that can be done.
First and foremost is to feed the little beggers less.  Second is to be
sure that the food you feed is appropriate for the fish you keep, with no
more phosphorus than they need and with that phosphorus in a form that's
available to the fish.  It might even help if you keep some growing and/or
breeding fish in the tank as long as you feed them with a sufficiently
high-calorie diet.  Live bearers seem to be great for that.

Roger Miller