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Re: Aquatic Plants Digest V2 #584

Sorry if this is a re- post but I havent seen it on the digest yet.

>From: George Booth <booth at hpmtlgb1_lvld.hp.com>
>Date: Mon, 17 Mar 1997 11:52:50 -0700
>Subject: Re: Buffers
>A few things may be going on here.  
>1) Your KH isn't high enough (you've already discovered this).  The
>CO2 controller can only lower pH.  If your "natural pH" with CO2 at
>equilibrium values (2-3 mg/l) isn't much over the set point, the
>controller can do very little about it.  We have enough KH in our
>water to give us a natural pH of around 7.8 and have the set point at
>6.9.  Thus, the loss of CO2 is mainly driven by diffusion into the
>atmosphere and the controller always has something useful to do.
>2) CO2 electrodes do rather poorly in "low ionic strength" liquids.
>Soft freshwater qualifies rather well as low ionic strength.
>Saltwater is high ionic strength.  I assume that Dupla did a good job
>in choosing the electrode for this situation.  I have found that the
>Broadley-James "Silver" electrode (Pet Warehouse, $36) seems to work
>the best of all that we've tried, even those costing over $100.

Thanks George, heres what has happened since: The Ph on Monday was now
holding steady at 6.75. As you can see the ph is slowly reverting to an
acidic state but it is however stable. The bicarb has obviously stabilised
the ph but why is it still decreasing in ph over time. This may suggest that
perhaps there is still more of an acidic buffer than an alkaline buffer. If
I were to add greater amounts of bicarb to the water this may fix the acidic
trend but result in a much higher dkh and therefore not have the desired
levels of co2. I cant for the life of me work this out. George, is it
possible that the dkh may decrease without water changes or does the bicarb
remain at the original concentration levels therefore always providing that
level of buffer. Is this making sense? I have eliminated the interference
with a grounding probe and did so some time ago. 

>> I had had a few problems with strange read outs after changing to
>> electronic ballasts
>3) Perhaps you're being plagued by induced voltages.  We've found that
>a grounding electrode in the water helps stabilize erratic pH
>Nope.  With enough KH and higher levels of CO2, the amount of CO2 the
>plants use and the variance in day/night respiration is a small part
>of CO2 loss.  Our tanks vary between 6.85 and 6.95. 

So given this explantation should I be aiming to have the total alkalinity
at such a level where the ph controller can manage the job it is required to
do, even if this means that the dkh becomes excessively high? What unit of
dkh do your tanks operate at? Do you only need to alter the kh after your
weekly water changes or is it something that must be done daily.

The trend to acidity does not sound right to me. Is it possible that there
is something else a-miss here? I presume from the above statement that the
process of photsynthesis is negligible on managing ph.