Re: CaPO3 solubility (fwd)

A friend sent me the following information about calcium phosphate
solubility and gave me permission to forward it.

>How soluble is CaPO3? If an aquarium has significant amounts of
>Ca ions will these precipitate the majority of phosphates? 

**Steve, I don't think so.  This is because the Ca and phosphate
concentrations in any aquarium water would not be high enough to initiate a
precipitation reaction.  Any precipiation is dependent on the concentration
of the individual ions (Ca and PO4) and the solubility constant.  CaPO4 is
not that insoluble.
     A much better source of CaPO4 is fishfood.  That is, the CaPO4 that
makes up the bones, teeth, and shells of fish and invertebrates that make up
fishfood.  As an ingredient of fishfood (ground fishmeal and shrimp), CaPO4
would be excreted by the fish in the feces.  This would collect in the mulm
and feed the plants.
     Phosphate is 'captured' by soil particles, though.  There are studies
showing that when phosphate is added to aquaculture ponds as a fertilizer,
the phosphate is removed from the water within a week or two.  The removal is
believed to be due to the strong affinity of soil particles for phosphate. 
This is a common well-known phenomenon-- the affinity of soil particles for

>If most of the phosphorus ends up as CaPO3 or FePO3 (precipitate), 
>is this a usable form for plants? 

**I believe it is.  Some P would inevitably be released from the CaPO4 or
FePO4 precipitates.  This P would probably be quickly snapped up by soil
microorganisms, soil particles, plant roots, and thus, and become part of the
organic cycling pool within the substrate environment.  At some stage, plants
should be able to dip into it.  The fact that it would probably be enough to
provide plants with adequate P is because there is so very much P in

>Papers I read refer to DRP or 
>dissolved reactive phosphorus so presumably some forms of phosphorus 
>are not reactive or available to plants.

**Yes, some (about half) of the P in soils is bound very tightly and not
immediately reactive.  

We went on to discuss the problems of P and/or Fe being the limiting
nutrient for algae and there seems to be some evidence that it's
really Fe in aquarium water that can trigger algae growth. Let me offer
some evidence. In one of my current tanks I have a rich substrate which
also has a layer of Fe rich clay. While the tank so far hasn't self-
destructed with H2S or anything else, it does have very high levels of 
nitrate and about .5 - 1 ppm of phosphate. The rooted plants were growing
fine and showing what I believe now to be a calcium deficiency. Floating
plants were really not reproducing at all. I added a dose of Flourish
and almost immediately the floating plants responded beautifully. A
few days later, the water got a greenish tinge which is getting a little

Incidentally, I sterilized the tank and all the plants before hand and
I do not have any black, thread, hair or other filament algaes. It is
necessary to do a complete treatment; simply dipping plants is worse
than a waste of time; it weakens them. You also have to be very careful
whenever you introduce any new plants. Sterilize them using the bleach
treatment. The same applies to equipment like nets. Don't add water
that comes with fish or plants. Remove tank water to a container and
use that to acclimatize the fish before netting them into your tank.
Paul recommends leaving them in quarantine for a few days to eliminate
algae spores in the fish feces.

I should add that too much P in your aquarium water can probably
also trigger some forms of algae so it's probably a combination of
P & Fe.

Steve in rainy old Vancouver getting ready for the weekend.
Have a good one!