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Re: KH drop means not enough CO2

On Sat, 6 Mar 1999, Karen Randall wrote:

> Alex Pastor wrote:
> >>The pH is 6.7 in the morning before the lights go on.  (3 watts/gal)
> >>It is up to 7.5 by the evening when the lights go out.
> <snip>
> >Carbonate hardness of the
> >>water I put in is 3, but when I measure it in the tank at the end of the
> >day
> >>it is maybe 1.5.  This concerns me. I assume it is because the nitrate is
> >>not zero.
> Russell Furlow wrote:
> >First off sounds like you may not need CO2.
> It sounds to me like he very much needs supplemental CO2.  The reason the
> KH in the tank is dropping is because the plants are meeting their carbon
> needs by splitting it out of the carbonate/bicarbonate in the water.
> Without correction, this will lead to larger and more dangerous pH swings
> as the buffering ability of the water is depleted.

When plants start using bicarbonate as their carbon source, I think the pH
is usually higher than the 7.5 max in Alex's tank.  Also, as near as I can
tell from studies on the mechanism for bicarbonate use, plants shouldn't
decrease alkalinity when they use bicarbonate to supply carbon.  The
bicarbonate is simply replaced with a lesser amount of carbonate that
creates the same alkalinity.  And the pH skyrockets - I've read pH over 9
in a couple of my tanks when the plants were really cooking.

I went back a few digests and reread some (not all) of Alex's posts. I
can't tell much for sure, but it did prompt me to suggest an alternative.

It sounds to me like Alex may be feeding the tank heavily - perhaps there
are a lot of fish in the tank.  The low morning pH indicates a fairly high
CO2 level, so in the absence of added CO2 there must be a lot of
respiration going on.  Even the 7.5 pH maximum doesn't indicate real low
CO2. In the absence of a lot of respiration the plants could pull the CO2
during the day down to near zero, and the pH would be much higher.  Heavy
respiration by fish and bacteria are required to produce those CO2
concentrations.  Alternatively, maybe it's moderate respiration and very
limited aeration or circulation.

The drop in KH from 3 at the tap to 1.5 in the tank is something I'd
usually associate with nitrification rather than with bicarbonate use by
plants.  Of course, that wouldn't be a night-to-day variation, but it
wasn't clear from some of Alex's other posts that the difference in
alkalinity was a night-to-day cycle.  If it is a daily swing, then it
might be kinda tough to explain on the basis of the info that Alex gave

I think I agree with Steve Pushak, that Alex should get better
circulation, complete with surface turbulence.  More frequent water
changes might also be needed to keep the KH up. Reduced fish feeding and
possibly a smaller fish population could be indicated.

If Alex does something to reduce CO2 from respiration, then the plants
will probably need added CO2 from another source.

Roger Miller