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Re: Carbon splitting plants
On Mon, 8 Mar 1999, Karen Randall wrote:
[snip]
> >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 what happens when the carbonates are exhausted?
I don't know that plants do use carbonate (CO3--), only bicarbonate
(HCO3--). I think when the bicarbonate is exhausted photosynthesis stops.
At that point, the pH is like 12 or something (as my 14-year old would
put it).
> I _know_ that in
> strongly lit tanks without supplemental CO2 (or even _with_ supplemental
> CO2 but not enough) the KH decreases over time.
In these cases I suspect that carbon and/or the energy necessary to get
bicarbonate converted to CO2 are growth-limiting. If so the plants
probably won't be consuming much of the available ammonium before it's
converted to nitrate; it's the nitrification process that lowers the
alkalinity. That happens in unplanted tanks all the time and can happen
in planted tanks as well.
Biogenic decalcification may also be playing a role. In that case the
alkalinity and general hardness are reduced by the precipitation of
calcium carbonate - often on the leaves of the plants. In natural water
bodies (as someone on the list observed recently) this is done by algae
and forms a calcareous muck on the bottom of hard-water lakes and ponds.
Biogenic decalcification will only occur when the calcium content is high
enough - it's a secondary process rather than an inherent part of plants
using bicarbonates for carbon. It's also a potentially reversible process
(calcium carbonate being redissolved at night) and could explain some
day-night cycling in the alkalinity.
I've never noticed calcium carbonate precipitation in my high pH tanks
with bright light and without added CO2 (in fact, snails have a hard time
keeping their shells on). My water carries very little calcium. I've
also never noticed a reduction in the alkalinity, but my alkalinity is
fairly high, I get a lot of evaporation and I change 15% of the water
weekly.
> In fact, Sera's whole
> campaign is to let plants take advantage of this, and rather than add
> supplemental CO2, just track KH and replace as necessary via their KH tablets.
Now there's an amusing ploy :) [further comments re Sera removed by
better judgement]
>
> If Alex's pH really _is_ going no higher than 7.5, then your explanation is
> certainly a strong possibility, although my preference would always be to
> lower fish and feeding loads and supplement CO2 rather than go to strong
> aeration. Particularly if you are serious about plants. High fish loads
> and feeding will inevitably lead to other problems down the line.
I agree entirely that high feeding loads are likely to cause other
problems. If high loads are the problem, then increased aeration may be a
stop-gap measure and reducing the feeding load is perhaps the best
solution. An intermediate solution might be to go to more frequent and/or
larger water changes.
So just what is a heavy fish load? My two 10 gallon, high pH tanks are
fed a small pinch of flake food once/day, and that's mostly to maintain
nutrients for the plants. The tanks each hold a pair of flag fish (and a
couple juveniles, now), a stable population of +/- ten guppies, a few
shrimp, pond snails and trumpet snails. I think that's a low to moderate
load. My "problem" tank is a 55 with what I call a moderate load (fed a
pinch of prepared foods twice a day plus the occasional chunk of
zucchini): 6 blue tetras, a lonely congo tetra, a cory cat (also lonely),
two SAE's, two clown pleco's and 8 or so adult blood red swordtails
(there's almost always a few baby swordtails there too, but I move them
out once they're catchable), ramshorn and trumpet snails. I have tanks
with light loads (one fish/20 gallons, fed a little when I think about
it), but I don't have anything I'd call a heavily loaded tank.
Anybody else want to put in on what they consider to be a
lightly/moderately/heavily load tank?
Roger Miller