Re: Biogenic Decalcification

> From: DAWB.DSKPO33B at DSKBGW1_ITG.ti.com (David Webb)
> I looked in The Optimum Aquarium yesterday evening to freshen up my
> knowledge after reading this post.  I didn't see anything that said
> the pH specifically goes either way, just that the KH drops to near
> nothing, making your tank susceptible to rapid pH swings.
> I can see situations based on this where the pH could either rise or
> fall, depending on other ions in the water.

On page 77, section 3.4.9, they lead into biogenic decalcification:

 "When the amount of free CO2 in water diminishes a process called
 biogenic decalcification begins.  Through this process the plants cover
 their need for CO2 by taking their carbon from the bicarbonates.  The
 pH value may rise to 9 or 10 during the course of this process". 
On page 79, section, they continue the biogenic
decalcification dicussion: 

 "If the amount of free CO2 falls, calcium precipitates.  This is a
 procedure which can be observed in the aquarium through calcium
 deposits around the ater surface, on the leaves, and in other places.
 This process is always accompanied with some rise in pH value."

Allgayer and Teton, in _The_Complete_Book_of_Aquarium_Plants_, allude
to this and mention: 

 "Traces of calcium carbonate on the aquarium walls and decoration
 indicate a shortage of carbon dioxide in the tank. To understand
 where these traces come from we have to revert to the following
    CaCO3 + H2CO3 -> Ca(HCO3)2

 This reaction will only take place where carbon dioxide is present.
 In its absence, it works the other way round.  The sodium bicarbonate
 yields carbon dioxide in teh water and the insoluble calcium
 carbonate forms a precipitate in the tank.  Under balanced
 conditions, where there is neither a deficit nor an excess, there is
 a permanent exchange between carbon dioxide and bicarbonate.  But if
 there is a shortage of carbon dioxide, the bicarbonate has to yeild a
 lot of thsi gas so that a large amount of insoluble carbonat will be
 formed - and as a result calcium deposits will appear."

Even though the text doesn't exactly match the equation, I think they
are saying that KH is decreased (which would cause pH to drop) but the
resulting free CO2 is also removed by plants and diffusion into the air, 
causing a net rise in pH.