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Re: Low pH problem

 

Wright Huntly responded to:

> > > 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...

Not the most precise of descriptions, but most "natural rock" is stable
and won't cause sudden, large changes in water chemistry.  The exceptions
are some quarried and mined rocks.  Other rocks will dissolve and 
increase water hardness or pH but usually (particularly with 
coarse-grained rock) this is a relatvely slow process.  Slow, that is, 
when compared to the effect from adding finely ground pure reagents with 
a similar composition. 

[snip]
 
> 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.

It would be a fairly extreme case.  Particularly since the low pH tends 
to suppress bacterial activity, including nitrification.  If your 
experience with very poorly buffered water indicates that nitrification 
isn't likely to cause acidification, then I'll defer.

> 
> > 
> > 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).

Alum ( K2(SO4)Al2(SO4)*24H2O ) isn't a naturally occuring mineral.  The
relatively rare mineral alunite (KAl3(OH)6(SO4)2) is somewhat similar and
can be used to make alum.  But if alum were present then certainly is
would make things acidic.  Muscovite and sericite mica are pretty stable
under near-surface conditions; neither contains sulfate and neither 
should alter pH. 
 
> 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.
> 

Pyrite (FeS2) in particular is quite insoluble, but its effect on pH does
not require its dissolution.  Both the iron and the sulfur are in reduced
states and can be oxidized without dissolution either spontaneously or
with the help of specialized bacteria.  The net reaction forms goethite
(FeO(OH)) and produces sulfuric acid.  Oxidation of sulfide minerals is
the reason that some mine drainage water can have a pH of 2. 

> 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!]

I'm impressed :-), but I still don't think it would be alum.  I agree 
that if it isn't nitrification then it is probably something in the 
substrate or the ornaments.

A short note on rocks as ornaments.  

Petrified wood probably is the safest ornamental stone for aquariums. 
Stream cobbles are usually safe, but if you get them from an area where
there are a lot of caves then some might be limestone or dolomite.  They
can raise your pH and hardness.  Most mined or quarried stones should
probably be avoided unless you can tell what kind of stone they are. 
If you can identify them, then you will find that many are safe.  For
instance, I used to have a 3 pound chunk of jade in one tank.  Jade is
safe, but its color clashed with the plants, so I took it out. 

Don't assume that buying ornamental stone in a fish shop means that it's
safe.  I've seen stores sell readily soluble rocks (specifically, selenite
gypsum) as ornamental stone.  I wouldn't be completely stunned to find a
fish store selling rock salt as an ornamental stone! 


Roger Miller