Re: KH and pH

[Dave Millman <dave at tactics_com>]

At 03:45 AM 3/28/2003, Jennifer Marshall wrote:


>I slept through my high school chemistry class, so please bear with me:

You spend your youth partying, we pay the price. I'd complain if I hadn't 
asked so many questions over the last couple years, and gotten so many 
great answers from generous people here and on 
rec.aquaria.freshwater.plants. So here goes.

>I have a 20gal plant/community tank with no added CO2.  Even with weekly 
>water changes, I was getting chronically low pH (6.2-6.6) and a KH of 1 if 
>any. I started adding a shmidgen (very scientific measurement) of baking 
>soda to each gallon of fresh water during water changes. This definitely 
>raised the pH; I'm now at a more comfortable level of 7.0-7.2, but my KH 
>is still a low 1-2 and continuously drops between the baking soda shmidgens

You are doing the right thing. Slowly increase the amount of baking soda 
you are adding on a regular water change schedule, so that your tank ends 
up around KH 3 or 4.

In my case, I add 1 level teaspoon baking soda to 20 gallons of change 
water per week to keep my KH constant.

>  The pH of my tap water is of course way off my chart, which ends at 7.6, 
> and the KH of my tap water is 3-4.  Just in case it's all related: the GH 
> of my tank is about 7-8 and that of my tap water is about  9-10.

Something is dragging your KH down from a tap level of 3-4. Questions:

    1. How frequent are your water changes, and what percentage?
    2. What is your plant load-what percentage of the gravel is covered 
with plants?
    3. What kind of plants?
    4. How many and what kind of fish?
    5. What else do you add to your water, if anything?
    6. Although this would drive KH up, do you have shells,
        beach sand, limestone decorations or anything similar?

>SO, my question is: what is it that makes the KH not stay constant? Would 
>the two very very small pieces of driftwood affect the KH as it supposedly 
>lowers the GH? Is there a more efficient way to keep the KH, and thus the 
>pH, constant? Does KH always fluctuate and I am just being neurotic?

With the answers to my questions above, we can help more. But in general:

    * pH is a function of KH and CO2: Increasing KH
      raises pH, and increasing CO2 lowers pH.

    * Stir tap water and let it sit overnight before testing it.
      Dissolved CO2 can lower the pH, and a lack thereof
      can raise it. Leaving it overnight will allow the CO2
      to reach equilibrium with the atmosphere, at about
      3ppm.

    * Normal processes in a planted tank slowly lower KH.
      Mine drops between 0.25-0.5 per week. Your KH is
      is so low that small drops like this can have a
      dramatic effect on your pH. Adding more KH
      increases the buffering capacity of the water so the
      slow decline in KH cause much smaller pH changes.

>I just got a 68gal tank and I am doing all kinds of research on using 
>carbon dioxide, but before I start using any, I want to make sure that I 
>have solved my KH/pH mystery.  Any advice?

You are asking all the right questions. If you inject CO2 to a tank with KH 
below 1 and falling, you will very quickly end up with battery acid and 
dead fish. The cause of your falling KH may be as simple as small water 
changes not replacing KH as quickly as natural processes consume it in your 
tank. Send us the answers to the questions above and we can tell you for sure.