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Re: The phosphate question revisited(sorry)

On Mon, 13 Dec 1999, Glen Williams wrote:
> Hopefully people are not bored of this subject but interestingly this was
> raised as a letter in Practical Fishkeeping. The writer of the letter did a
> test where he placed some food into phosphate free water. The phosphate went
> off the scale on the test kit he was using.
> Practical Fishkeeping asked a "major" manufacturer of food why this was.
> Apparently (and you can tell I am certainly no expert) phosphate is
> contained in all fish. Therefore the reason that it is added to the food is
> as a good supplement. 

The answer does sound a little flippant.

The manufacturer was right in the sense that phosphorus is essential to
and a part of all living things, including fish.  I understand that fish
can use phosphorus directly out of the water, but I don't think they can
usually supply all of their phosphorus needs that way.  So, they must have
phosphorus in their food.

How much phosphorus and what form of phosphorus is another matter
entirely.  Hagen (on their web site) claims that fish need 0.5% to 0.9%
phosphorus on a dry weight basis in their diets.  Manufactered fish foods
are formulated to contain at least enough available phosphorus to meet the
fish's needs.  The fish foods also contain additional phosphorus that
isn't available to the fish; the unusable phosphorus increases the total
phosphorus content of the food without increasing the phosphorus value of
the food.  In addition, some manufacturers add inorganic phosphorus to
some foods; some forms of soluble inorganic phosphorus are available to at
least some fish.

Very little of the phosphorus in fish foods should be immediately released
into the water and measurable with a test kit.  The phosphorus should be
contained mostly in biochemicals (like DNA, ATP, phospholipids) or
inorganic solids that wouldn't be immediately measurable.  If it is
immediately measurable it implies either that a lot of soluble inorganic
phosphorus was added to the food (check the contents) or that some of the
ingredients were partly decomposed before it was put into the fish food.
That's a low-quality fish food.

Perhaps the question to the manufacturer should not have been why the
phosphorus was there, but why it was so quicky measurable in the water.

Roger Miller