Re: driftwood & pH

Matt Van Buskirk wrote:
> Could a piece of driftwood in my 135 gallon tank be buffer my pH at 
> 7.6?  After replacing my gravel 6 months ago after finding out it was 
> causing my pH and hardness to rise, I haven't been able to get my pH 
> down even after bubbling CO2 into the return line from my filter.
> I purchased an RO unit which is probably one of the best investments 
> you could do if you live in a state that has as hard of water as mine 
> does (Utah).  I got the hardness down from over 30 ppm to about 6-7 
> ppm by diluting the tap with RO water.  The pH of the tap is 7.6 
> and when mixed with RO it remains at 7.6.  
> My limited understanding of the chemistry involved leads me to two 
> possibilities
> 1-  not nearly enough co2 is being dissovled in the tank, so increase 
> the amount bubbled in the return line  (an idea I don't like because my tank houses 4 
> almost full grown discus who at last if I remember breath oxygen and 
> not carbon dioxide), or hook up in my tank the ugly co2 diffuser 
> that came with my sandpoint co2 tank to try and diffuse more of the 
> gas that is being let out. (those of you who have one know 
> what a big green ugly monster it is, not to mention you have to hook 
> up a powerhead to it)
> 2- something is buffer my water.  I tested the gravel extensively, it 
> would raise RO water to about 6.8 and then stop, the only other 
> object in the tank is a big piece of driftwood?  I got the impression 
> that a decaying piece of driftwood would actually lower pH.

I suppose it depends upon what sort of chemicals have been impregnated
into the driftwood. If it had been soaking in an alkaline pond or in
sea water, it could have lots of alkilinity. I would suggest that you
test a piece of it in RO water and see what it does to the pH. If it
changes, you should probably seal the drift wood with urethane or
something. I think commercially sold drift wood has had that done.

Do you know the concentration of CO2 in your tank? For plants you should
be aiming at 15ppm. If you have specific pH requirements for the Discus
then you need to adjust the amount of alkilinity and carbonate buffering
to achieve that. If you are mixing tap and RO water, you should be able
to achieve very soft water which you will need; you will need to use
more RO water than tap water I suspect. It is sometimes difficult to
determine the relative proportions of the hardness components of
water, so you may prefer to simply add sodium bicarbonate and calcium
carbonate to RO water to establish your precise hardness requirement.
What is your target pH? from the kh.co2.ph table assuming 15ppm we find 
that at pH 7.0 the kH is 5.0. For pH of 6.6 a KH of 2.0 is required.
That's very soft water.

Anyone care to give us a formula in grams per liter and tsps per gal
for an acceptable ratio of sodium bicarbonate and calcium carbonate
when added to distilled water to achieve 1 degree of KH? We should
add this to our FAQ. I know it's been mentioned before too.

If you are pretty certain you have the carbonate hardness correct and
the pH is still not correct, then certainly the required CO2 is not
present. If you are bubbling the CO2 into the return of the filter,
this is probably the problem. Try putting it into the inlet. Don't be
concerned about getting 15ppm of CO2 in your tank; this will not
create any problems for the Discus; dissolved CO2 has no effect upon
the amount of O2 which can be dissolved in water. It is also well
below the level which would create a problem for the Discus to
eliminate CO2 from their blood (+30-40ppm I'd guess).

The pH of your tap water will not change (significantly)when you add 
RO to it because the RO has no acid or base to alter the pH however
with a lower hardness, the pH will change much more easily with the
addition of CO2.