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On Fri, 18 Feb 2000, Chuck Gadd wrote:
> Today after checking the pH of the 75g (6.8 @ 3dKH), I checked the
> pH of the 29g. Well, it was low. About 6.6. But I'm not adding
> any CO2 to that tank. Well, I just measured the KH, and it's
> less than 1dKH. I assume it's something like biogenic decalsification.
> But reading on The Krib, I see a bunch of messages talking about rising
> pH from biogenic decalsification. So, which is it? It would seem to me
> that if the KH drops off, the pH should fall.
Plants don't use much bicarbonate as a carbon source until after the CO2
supply is already depleted and the pH is high. Biogenic decalcification
occurs when the pH is so high that calcium carbonate is precipitated.
Under those conditions the pH is frequently over 9.
> I haven't been good about doing frequent water changes on the 29g. How
> long should it take plants to strip out 3dKH? I've got a spare 10g tank
> set up with no fish, no CO2, and moderate light, and a bunch of clippings.
> I filled this tank with 100% fresh tapwater just a week ago, and it's
> KH is already less than 1.
In the case of the 29 gallon tank, nitrification might be a more likely
cause for the drop in alkalinity than biogenic decalcification.
When ammonia is nitrified, the overall reaction is something like:
NH3+2O2 -> NO3- H+ + H2O
and the H+ reacts with the buffer:
HCO3- + H+ -> CO2 + H20
Each 17 milligram of NH3 that's nitrified can destroy 61 millgrams of
bicarbonate. If you don't change the water, then nitrification can rather
quickly consume all of the alkalinity in poorly buffered water.
If you fertilized the 10 gallon tank with a nitrogen fertilizer with
ammonia or urea then nitrification might explain the drop in KH in the 10
gallon tank. In my experience, plants don't use much bicarbonate unless
the light is fairly bright, so if you didn't add ammonia or urea you might
need to look for some other acidifying influence that could drop the Kh.
> Is there something I should be adding to the tank to keep the KH up?
> If I do more frequent water changes, I'm assuming the plants will keep
> pulling carbonate out of the water, right?
Water changes are the best way to do it.
> Does all this mean that a tank with live plants and lots of light
> NEEDS CO2 to prevent dropping KH? I was always under the impression that
> a planted tank would run just fine without CO2, just with slower growth.
Aside from any problem with KH, if you don't add CO2 to a brightly lit,
heavily planted tank then the plants that are most well-adapted to use
bicarbonate and live with high pH and no CO2 will outcompete all of the
less well-adapted plants in the tank. Most of the plants will die and you
will end up with a tank full of one kind of plant (e.g. jungle Val).
Planted tanks can do OK without added CO2, but only with moderate to low
light. Bright light and no CO2 creates a hostile environement.
I think that the most important thing we do when we add CO2 to a planted
tank is reduce competition and allow more plants to coexist. The CO2
doesn't need to be in the "optimum" (see Steve, I can use that word too!
Do you think I'll get away with it?) range to do that, it just needs to be
there all the time. Even 4-5 ppm CO2 maintained all the time makes a
remarkable difference in brightly lit tanks.