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Re: Low pH problem

"Roger S. Miller" <rgrmill at rt66_com> wrote:

> Subject: Re: Low pH problems
> J. Daniels wrote:
> > I'm looking for some input on how to fix my low pH problems. I've got a
> > 108 gallon tank with natural rock and is fairly heavily planted with a
> > low fish load (about 25"). The pH is consistently between 4.5 and 4.7.

Ah, yes. Natural rock...

> [snip]
> Actually, I'm a little surprised that you have anything alive in the tank
> at that pH.

Naah! Those of us who keep "wild" Betta species and killies routinely
keep both fish and plants (if you call mosses and ferns "plants") at
those acidic levels. :-)

> Check the buffer capacity in your tap water.  It must be very low.  There
> is no buffer capacity in your tank water at a pH of 4.5.  It appears to
> me that something in your tank is producing a strong acid and that is
> wiping out whatever small buffer capacity your tap water provides.
> I can think of two sources of strong acid.  The first is the nitrification
> process.  Nitrifying bacteria are a constant source of acid, and the more
> you feed the more acid they produce.  Most of us have tap water with
> enough buffer capacity that regular water changes are all we need to
> offset the pH drop from nitrification.  But, if your tap water buffer
> capacity is very low then it won't take much nitrification to drop the pH.

I've never been able to get even RO water to go that low on simple
nitrification processes, alone. YMMV. The fish *I* have show the bad
effects long before it gets there.

> The second alternative is something I've never read speculated about here,
> and I'm not one to let a good speculation just go to waste.  If your
> natural rocks include sulfide ores (most likely pyrite - same as fools
> gold) then oxidation of the pyrite will produce acid.  Again, if your tap
> water buffer capacity is very low, then it wouldn't take very much acid to
> drop your pH.

What is much more probable, IMO, is that the tank contains sulfates
rather than sulfides. Alum occurs naturally in a variety of rocks, even
volcanic ones. It also can be very high in pyrophllitic clays (see
muscovite and sericite mica, for example). As the aluminum-sulfate part
steals an OH- to become insoluble AlOH, the left-over H+ from that water
molecule plus water and the sulfate ion can make sulfuric acid. There's
good reason this stuff tastes so acid, as well as being bitter!

Sulfides, like iron pyrite, have vanishingly small solubility, and I
don't see any ready oxidation mechanism for them as they are already
just about as oxidized as it ever gets.

My vote is that something in the ornamentation or substrate is taking
advantage of the limited buffering to cause the pH to drop, but it's
more likely to be some version of alum.

See? I can speculate with the best of you :^) [Waste not, want not!]


Wright Huntley, Fremont CA, USA, 510 494-8679  huntley1 at home dot com
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