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Re: [APD] 0 KH

Just a few random thoughts here on the topic.
I think the process of buffering is poorly understood by many. If by buffer, one means counteracting another agent then a carbonate or bicarbonate does that in a planted aquarium-adding some will tend to move pH in the opposite direction of CO2 in water. 
If by buffer one means eliminating or reducing changes (as you add more or less CO2 the pH tends to remain constant), then that's not something you're going to see in a planted aquarium as a result of carbonates or bicarbonates. I think many folks think tank carbonates buffer in the second sense in a planted aquarium. That it reduces swings in pH as more or less CO2 is in the water.
You *can* buffer water, in the second sense, with, for example, calcium carbonate (CaCO3) if you add enough that some remains *undissolved* and the pH, therefore, will be around 9-10 units, iirc. As long as some CaCO3 remains undissolved, you can add more acidifier, such as CO2, as the added acid uses up some fo the dissolved carbonate, more of the undissolved carbonate dissolves into the water until the precipatation pH level is reached. This is a continuous and cynamic process, so the pH tends to remain constant -- it doesn't swing. In the reverse direction, less acidifier will cause more CaCO3 to precipitate out of solution and the pH will remain constant.  But few aquatic gardens are maintained at around pH of 9 or more.
So, in a planted aquarium, adding carbonates is just a way to offset the acidifying effect of adding CO2; the presence of dissolved CaCO3 won't hold the pH constant or reduce tha mount of change in pH caused by subsequently adding or subtracting of CO2 .
I think the two different senses of "buffer" get used somewhat interchangeably, sometimes by the same speaker, and this leads to some confusion.
Another point, some plants can use carbonates as a source of carbon -- Vals are a classic example-- but they are not likely to do much of this if there is plenty of CO2 available.
So the main value of adding carbonates is to move the pH up from where it would be without the carbonate.
At very low pH levels, say around 3-5, the bacteriological acitivity might be significantly reduced, which could be critical in a plantless aquarium but is less important in a planted aquarium. For one example, the Rio Negro has a pH roughly around 4, very little mineral content and very little bacterialogical activity. Fallen-plant detritus colors the water but doesn't provide much inthe way of plant nutrition for aquatic plants. The river is absent aquatic plants except in a very few places, and usually not submerged.
But that's a river and not a glass box into which we can add plenty of nutrients.
Do most plants care about the pH? Well, the info we usually see is about the conditions where the plant grows natively, which is not necessarily the same as what it prefers or what it will tolerate. It is interesting to find out these last two.
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----- Original Message ----
From: Vaughn Hopkins <hoppycalif at yahoo_com>
To: aquatic plants digest <aquatic-plants at actwin_com>
Sent: Wednesday, July 5, 2006 11:10:07 PM
Subject: Re: [APD] 0 KH

I was adding bicarbonate of soda when I did water changes, but I have  
stopped that.  I don't see a difference in the tank yet, but it has  
been less than a month that I've been doing that.  My tap water runs  
around 2 KH.  I had been trying to achieve the 3-4KH minimum I  
understood to be essential.
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