Battle with pH and kH! - The Planted Tank Forum
 
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post #1 of 15 (permalink) Old 11-26-2013, 10:03 AM Thread Starter
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Battle with pH and kH!

Hey guys,

I am new to closely monitoring water parameters, and let me tell you, I am having some trouble.

I am at a stage in my tank where I am just trying to grow a carpet and cycle my tank. I've been advised that maintaining a roughly neutral pH and a kH of at least 3 degrees is helpful for this.

Without intervention, my pH is 6 and my kH is 0... I added bicarb soda to get these parameters to where I want them. I am using pressurised CO2 as well, at a rate of 1-2 bps.

After two days though, they've returned to what they were originally. Of course I can keep adding bicarb every few days, but is that entirely necessary? Should I have to? Is there a more permanent way to increase these parameters?

I hope these aren't entirely daft questions...

Thanks in advance for any help or advice you can give! =]

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post #2 of 15 (permalink) Old 11-26-2013, 11:15 AM
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Don't bother battling it. Fighting your water is a fruitless endeavour. At this point you should be running your co2 as high as possible.


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post #3 of 15 (permalink) Old 11-26-2013, 12:39 PM
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Yes, keep adding the baking soda or any other form of carbonate. Potassium bicarbonate is good.
The nitrifying bacteria need the carbonates.

After the cycle is complete you can let the tank revert to what it wants to do, if that suits the fish you will be getting.

Sounds like you have a substrate that is removing the KH. Is it one of the ADA products, or Turface, Safe-T-Sorb or a related material?

If you need a permanent solution, and will be keeping hard water fish then you can blend that substrate with coral sand, limestone sand, oyster shell grit or similar material. I did that with Turface (roughly 50% Turface 50% coral sand) and it stabilized the KH nicely.

For a temporary solution put any of those materials in the filter in a nylon stocking. When the tank is cycled remove them.
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post #4 of 15 (permalink) Old 11-26-2013, 01:04 PM
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Do you happen to know your GH? It quite possibly needs adjustment as well. It's possible that your substrate may be affecting the KH as well as bacteria and other factors. Sodium bicarbonate isn't very stable so it changes things fast. There are commercial products to raise KH that are more stable. You can also use natural products like aragonite, crushed shells etc. although these will raise your GH.
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post #5 of 15 (permalink) Old 11-26-2013, 03:11 PM
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A KH of zero is very unusual unless you use distilled water, or rain water. Well water almost has to contain dissolved carbonates, so it has to have a KH above zero. Are you sure your KH test kit is working well? Many plants do best with low KH, so I don't believe there is a good reason to raise the KH as long as you have a measurable KH, and once you start doing that, you have to always try to match the tank KH when you do water changes. Making big changes in KH quickly is not good for the fish. The pH of your water isn't a critical parameter.

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post #6 of 15 (permalink) Old 11-26-2013, 07:08 PM
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A kH test works in a very indirect fashion.

The first reagent bottle is essentially a pH indicator, much like in a pH test kit. The second is an acidic solution. Since kH resists downward shifts in pH, the test measures kH by seeing how many drops of acid it takes to lower pH to a specific point.

However, dissolved CO2 is also acidic. So if you kH test water with CO2 in it, you will get a falsely low result.

Try putting a bit of tank water in a bottle with mostly air. Close the lid and shake the heck out of it. This will drive off most of the CO2. Test that water for a more accurate result.

This adds to, rather than invalidating, other comments you've received. As [Diana] said, it's good to have some carbonates. As [Zorfox] said, if you have a reactive substrate, it could be soaking up your carbonates - but that's reversible and it can also release them as needed. Or it could be releasing another acid which interferes with the test. And as [xmas_one] said, fighting water parameters too much is fruitless. You don't want pH dropping too low, but 6.0 is acceptable, and there's rarely any valid reason to maintain it at 7.0.
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post #7 of 15 (permalink) Old 11-27-2013, 12:35 AM
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Quote:
...and cycle my tank...
Here is the current concern. Nitrifying bacteria use the carbon from carbonates. During the fishless cycle you are trying to get the bacteria to grow at its best rate. For this to happen they need carbonate.

If the substrate is removing the KH from the water (ADA materials, Safe T Sorb, Turface... will do this) then the tank will not cycle. Been there, done that.

Adding baking soda to keep the KH at least 3 degrees will do the trick.

When the cycle is complete, you can go back to water with no KH if that suits the fish you want. Otherwise, do something about it. Natural materials suggested above will indeed raise KH and GH. Baking soda can be used with fish, plants. Potassium bicarbonate will also do the job.

When the tank is fully planted the plants play a major role in removing ammonia etc. But there should also be nitrifying bacteria.
This is nowhere near the worry of a non-planted tank, of course. Unless the plants are not thriving.
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post #8 of 15 (permalink) Old 11-27-2013, 03:20 AM
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Quote:
Originally Posted by DarkCobra View Post
A kH test works in a very indirect fashion.

The first reagent bottle is essentially a pH indicator, much like in a pH test kit. The second is an acidic solution. Since kH resists downward shifts in pH, the test measures kH by seeing how many drops of acid it takes to lower pH to a specific point.

However, dissolved CO2 is also acidic. So if you kH test water with CO2 in it, you will get a falsely low result.

Try putting a bit of tank water in a bottle with mostly air. Close the lid and shake the heck out of it. This will drive off most of the CO2. Test that water for a more accurate result.

This adds to, rather than invalidating, other comments you've received. As [Diana] said, it's good to have some carbonates. As [Zorfox] said, if you have a reactive substrate, it could be soaking up your carbonates - but that's reversible and it can also release them as needed. Or it could be releasing another acid which interferes with the test. And as [xmas_one] said, fighting water parameters too much is fruitless. You don't want pH dropping too low, but 6.0 is acceptable, and there's rarely any valid reason to maintain it at 7.0.
My API KH test kit is a one reagent test. You add drops of the reagent, the first drop usually making the solution one color, and the last drop changing the color to another color. Each drop that take equals 1 dKH of carbonate hardness. I have never seen dissolved CO2 affect the KH reading.

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post #9 of 15 (permalink) Old 11-27-2013, 05:11 AM
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Quote:
Originally Posted by Hoppy View Post
My API KH test kit is a one reagent test. You add drops of the reagent, the first drop usually making the solution one color, and the last drop changing the color to another color. Each drop that take equals 1 dKH of carbonate hardness. I have never seen dissolved CO2 affect the KH reading.
I used the API KH test kit too. I'm probably mistaken about it being two bottles, as I stopped buying KH tests many years ago. As rarely as I did KH testing, I never got much use out of them before they expired and failed.

But my memory is absolutely clear that it was affected by CO2. Because I wondered where my KH was going too. I'd test 8°KH in aged tapwater. Do a 50% water change with tapwater, and read 0°KH in the tank the next day. And that was in a tank with gravel. Impossible! But shaking the water sample in a bottle with air first restored the KH.

Just to double-check, I looked up API's MSDS. Unfortunately, it doesn't list any ingredients; as listing anything that presents minimal hazard is optional. Looking for the first MSDS with a full listing, I found this:

http://www.microbelift.com/files/151...H_Test_Kit.pdf

The ingredients:

Bromocresol Green, Sodium Salt (according to Wikipedia, a pH indicator)
Hydrochloric Acid
Water


Which is exactly as I originally described, nothing more than a pH test with acid mixed in. And will be affected if another acid is present, like dissolved CO2.

As for why you haven't observed this effect, I'm unsure. Perhaps we performed the test a little differently. From the API directions:

"Cap the test tube and invert several times after each drop."

I did as directed, and inverted - which is just turning it upside down and then righting it. This does a fair job of mixing the liquids, but without significantly driving air into it and so results in minimal outgassing. Maybe you shook instead, outgassing most of the CO2 to the airspace in the test vial?
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post #10 of 15 (permalink) Old 11-27-2013, 08:18 AM Thread Starter
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Thanks so much, guys! You're really helping, and it's making much more sense to me now.

My substrate is CalAqua black earth. Do you guys know if this absorbs carbonates?

I don't know my gH, to be honest.

And with my kH, I tested it after the light and co2 had been off for some time, and it had a kH of 3 degrees. I just tested it again after the co2 has been on for a some time, and it was still three degrees. Promising. The other day though, I tested the kH two days after I adjusted it with bicarb, and it only took one drop to turn the vial yellow. So to correct myself, maybe it was a kH of between 0 and 1 degree haha. I only added bicarb yesterday, so the time period isn't the same between tests.

I suppose I'll keep adding bicarb then! I do think it's creating sediment on the bottom of my tank though:



Can anyone verify if the bicarb is what's caused this? Thanks!!

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post #11 of 15 (permalink) Old 11-27-2013, 03:04 PM
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Quote:
Originally Posted by DarkCobra View Post
I used the API KH test kit too. I'm probably mistaken about it being two bottles, as I stopped buying KH tests many years ago. As rarely as I did KH testing, I never got much use out of them before they expired and failed.

But my memory is absolutely clear that it was affected by CO2. Because I wondered where my KH was going too. I'd test 8°KH in aged tapwater. Do a 50% water change with tapwater, and read 0°KH in the tank the next day. And that was in a tank with gravel. Impossible! But shaking the water sample in a bottle with air first restored the KH.

Just to double-check, I looked up API's MSDS. Unfortunately, it doesn't list any ingredients; as listing anything that presents minimal hazard is optional. Looking for the first MSDS with a full listing, I found this:

http://www.microbelift.com/files/151...H_Test_Kit.pdf

The ingredients:

Bromocresol Green, Sodium Salt (according to Wikipedia, a pH indicator)
Hydrochloric Acid
Water


Which is exactly as I originally described, nothing more than a pH test with acid mixed in. And will be affected if another acid is present, like dissolved CO2.

As for why you haven't observed this effect, I'm unsure. Perhaps we performed the test a little differently. From the API directions:

"Cap the test tube and invert several times after each drop."

I did as directed, and inverted - which is just turning it upside down and then righting it. This does a fair job of mixing the liquids, but without significantly driving air into it and so results in minimal outgassing. Maybe you shook instead, outgassing most of the CO2 to the airspace in the test vial?
Very interesting! Here is the pH range of Bromocresol green:


So, we add one drop of the solution to 5 ml of water, and the acid in it drops the pH to below 5.0, turning it green, which is so far below that of the carbonic acid that the CO2 in the water forms, that it will have essentially zero effect on the pH. (Because pH is a logarithmic scale, not a linear one.) We keep adding drops until the color changes to yellow - the pH drops to 3.8. The number of drops needed depends on the KH of the water, thus making it a test for KH. I am almost sure the carbonic acid doesn't affect the KH reading. (I'm almost never 100% sure of anything.)

When I was doing my testing of KH I just inverted the tube once, since that was all the mixing it required to get the color to show up.

Hoppy
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post #12 of 15 (permalink) Old 11-27-2013, 04:45 PM
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Ooo, this is getting really intriguing!

Now I'm not 100% sure about this either. But in a tank, once carbonic acid concentration is high enough to exhaust the buffering capability of the carbonates, addition of more acid causes a rapid pH shift. I assume this applies to other acids as well.

So although carbonic acid cannot change lower the pH enough to turn Bromocresol Green to yellow on its own, wouldn't it still use up some of the buffering capacity of the carbonates; allowing the hydrochloric acid then to more rapidly cause the larger shift required to achieve the color change, with less drops?

Unless there's some caveat to the whole pH relationship I'm not aware of.
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post #13 of 15 (permalink) Old 11-27-2013, 08:44 PM
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I got hold of a KH test and tried to replicate my earlier result. Guess what?

I couldn't replicate it at all. Granted the highest CO2 ppm tank I currently have is only ~15ppm, so I can't exactly duplicate my earlier circumstances. But I couldn't get any difference whatsoever by offgassing the sample. Tried many times. Nada.

So I guess something must have gone wrong way back when, though I don't know what. And I'd still love to understand exactly why CO2 doesn't affect the test, it really seems like it should, at least a little.

But at any rate - disregard my previous advice.
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post #14 of 15 (permalink) Old 11-27-2013, 08:57 PM
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I think the answer to the question about why the carbonic acid doesn't use up the buffering capacity of the KH is simple: KH is not a buffer! Buffering refers to a mix of two salts that make the resulting solution maintain a specific pH when a little acid or alkaline substance is added. What KH is is a measure of how much carbonate ion concentration there is in the water. When CO2 is added to the water, the CO2 dissolves into the water and a relatively small portion of the CO2 forms carbonic acid, which balances the carbonate ions, thus lowering the pH. When you add an acid to the water it balances more of the carbonate ions, still further lowering the pH. There is no plateau where the added acid (or CO2) doesn't drop the pH, and no point where the added acid (or CO2) results in a disproportionate drop in pH. Since my chemistry background is very skimpy I recognize that I may have this totally wrong.

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post #15 of 15 (permalink) Old 11-28-2013, 08:03 AM
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Quote:
Originally Posted by Hoppy View Post
I think the answer to the question about why the carbonic acid doesn't use up the buffering capacity of the KH is simple: KH is not a buffer! Buffering refers to a mix of two salts that make the resulting solution maintain a specific pH when a little acid or alkaline substance is added... There is no plateau where the added acid (or CO2) doesn't drop the pH, and no point where the added acid (or CO2) results in a disproportionate drop in pH.
Thanks, that makes perfect sense. Carbonate certainly does not tend to hold a fixed pH as carbonic acid is added, as one should expect from a buffer.

Reading up on buffers a bit, I see my understanding of them was overly simplified. Will have to try and comprehend that better. In the meantime, I'll explain it to myself as only the stronger acid having any effect in this particular instance.
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