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Re: [APD] pH control

I think that many folks are missing the point.  An RO unit does nothing to
the pH as such.  A microporous membrane partitions dissolved materials,
basically by their sizes.  The residual mineral profile of the water, plus
the effects of dissolved gases in equilibrium with air, determines the pH of
the water. Carbonates and bicarbonates are the natural buffering system of
water in the wild.  We measure those as KH.  Nitrification uses up KH, part
of the normal process, so KH declines in any tank with a functioning
biolfilter.  That means than any tank will become more acid with water
partials to restore lost KH or supplements of carbonate/bicarbonate.
Mineral acids also destroy KH.

The normal state of any captive water volume with any enclosed biology is to
lose KH and thus reduce pH.  The pH is in and of itself showing nothing
beyond the balance between on-going biological processes and the original or
supplemented KH of the solution.  The pH is at most a symptom, describing
nothing of the processes going on, only their effect on one easily measured
parameter.  Modifying that parameter, pH, is not an effective solution to
holding water parameters relatively stable, especially if it is done by most
commercial buffer products which tend to be short-lived themselves.  If you
want stability in you water conditions, you are best off working with the
most stable buffering system available, the carbonate/bicarbonate system,
which is what is found in the wild.

But do remember that the pH does not really matter to the fish, the total
mineral profile of the water matters a lot.  Fish do not read external pH;
they read particular ions and total osmolarity.  At the extremes of
freshwater - tropical rainforest blackwater and the African Rift lakes - the
fish are adapted to water with very low TDS much of the year, with increases
during the dry season in the first case, or remarkably stable high TDS in
largely landlocked high water volume lakes.  Most fish are highly adaptable
to TDS/osmotic changes provided that they don't happen too fast, giving the
fish time to adjust own processes internally.  Even the Rift Lake fish can
and do adjust, showing that they underlying mechanisms (needed for the
riverine fish from which they evolved) have not been lost in the geologic
time during which they evolved in the Rift lakes.  In captivity, the biggest
issue is breeding certain blackwater fish where their egg membranes are
sensitive to GH (calcium and magnesium), which has nothing whatsoever to do
with pH.

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