C02 and the KH/PH chart - The Planted Tank Forum
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post #1 of 17 (permalink) Old 03-13-2006, 02:56 PM Thread Starter
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C02 and the KH/PH chart

I was re-reading Chuck Gadd's "Measuring CO2 levels in a Planted Tank," and ran across this statement:
Quote:
If your water contains high levels of phosphates, it will alter your water properties, and invalidate these CO2 calculations.
How true is this? I'm assuming this would also apply to the phosphates we add and just how high is "high?" I've been trying to keep mine at >2ppm which has done wonders in limiting Green Spot Algae, but do I not have anywhere near the levels of CO2 I thought I had? I'm keeping the KH buffered to around 3-3.5 (54-63 ppm) with a fairly consistent PH of 6.4 or so (best I can tell, I'm beginning to think I'm color blind . ) According to the chart that would indicate CO2 levels in the 35-40 range. I still get "fuzz" algae and/or hair algae (I think.) I did the Excel thing which almost eradicated it, but I'd rather not do it again. The fish were not happy campers although the shrimp acted like they could care less. I'm suspecting that I may not have the CO2 I think I have. I have my spraybar pointed slightly toward the surface to produce a barely noticeble surface ripple across 1/3 of the tank. But I would not think that would outgas a huge amount of CO2. I'm not using my controller at the moment (need a new probe as related elsewhere) and am trying the seat of the pants method that seems to work for other people. Perhaps 2bps and on 24/7.

Or am I way off base here?

Sláinte!
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post #2 of 17 (permalink) Old 03-13-2006, 03:18 PM
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I don't know how high PO4 levels have to be before they affect the kH/pH relationship, but they have to be way, way higher than 2ppm, I've never heard about that being an issue at the levels we keep PO4 at.

Sounds like your CO2 is at a pretty good level, but you could increase it a little, last time I checked mine was at 86ppm .

The kind of algae that you have doesn't sound CO2 related to me, but of course that's pretty much a guess since I don't know a lot of the details of your situation. How are you dosing? What are your NO3 levels? If your signature is accurate, it looks like you have a low bioload, so that shouldn't be a factor.

-Craig
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post #3 of 17 (permalink) Old 03-13-2006, 04:07 PM Thread Starter
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This is what I'm dealing with. It obliged me by popping up on a piece of dwarf sag close to the glass so I could get a decent pic of it:



Dosing is EI with Greg Watson ferts. Sort of . I've been testing almost daily trying to get a handle on how much of the various nutrients the tank is using to see if I'm putting in too much, not enough, or on the money.

1/4 tsp KNO3 (for Nitrates) - 3x weekly (was 1/2 but that seemed to be too much, it was keeping NO3 readings at 20-40 ppm) This seems to keep the readings at 10-20ppm.
1/8 tsp KH2PO4 (for Phosphates) - 3x weekly
1/2 tsp K2SO4 (for Potassium) - 3x weekly (I was getting holes in my stargrass. This seems to have stopped it.)

3 ml Flourish FE (for Iron) - daily
10 ml Plantex CSM+B (for Traces) stock solution (1 TBS per 250 ml water) 3x weekly alternating from macros

50% water change weekly.

Sláinte!
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post #4 of 17 (permalink) Old 03-13-2006, 06:22 PM
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I would go back to 1/2 teaspoon KNO3 3x a week and ignore the test kit.

Also, how long has the tank been running? The first time I set my 75 up, I got that same algae all over my glosso. I had some BGA start to grow too so I dosed some Boyd Enterprises Chemi-Clean that I had left over from my reef (antibiotic), it was probably a coincidence, but that algae that you have went away along with the BGA. I don't want to get flamed for using meds, it's not my preferred method, one definitely wants to ensure that all the nutrients are in check. It sounds like you are doing everything right, I would bump the KNO3 up, manually remove as much of that stuff as possible (toothbrush will work well) and ride it out. That stuff is one of the easier algaes to beat.

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post #5 of 17 (permalink) Old 03-13-2006, 07:22 PM
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You need a lot of phosphate in your tank before it significantly starts to interfere with the pH/KH chart.

This may help to make things clearer.
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post #6 of 17 (permalink) Old 03-13-2006, 07:30 PM
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It's not just phosphates that can invalidate the chart. Any other acid that is present in the water besides CO2 will throw it off too. This includes tannic acids from driftwood and acid produced from the breakdown of fish waste. These 'organic' sources of acid can add quite a bit.

There has been a lot of buzz on APC about this. Try these links:

http://www.aquaticplantcentral.com/f...?highlight=co2

http://www.aquaticplantcentral.com/f...?highlight=co2

http://www.aquaticplantcentral.com/f...?highlight=co2

http://www.aquaticplantcentral.com/f...?highlight=co2

If you don't account for the other acids present you'll underestimate the amount of CO2 you're putting in.
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post #7 of 17 (permalink) Old 03-14-2006, 12:29 AM
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You can try this to determinate CO2 vs any buffers

Hanns-J Krause CO2 Test
Author Handbook Aquaria Technology
Krause recommends a different method to measure CO2 content. That method is insensitive to the presence of unusual buffers. Not absolute pH is the key, but the change of pH by two units is used to determine correct CO2 concentration.

Step 1:Take a sample of your water and aerate it for some time until all CO2 is removed. In that case the concentration of CO2 is in equilibrium with the surrounding air (0.6ppm).
—> Measure pH of the water (=X).

Step 2: Stick a straw into the water and exhale through the straw into the water sample for two or three minutes. This sets the CO2 level at 60ppm.
—>Measure pH of the water (=Y).

The optimum CO2 concentration of 10-20ppm is at the pH value about 2/3 of the difference between X and Y:
pH optimumt = X +.67*(Y-X).

This will work even with buffered water, although the change in pH might be small and only detectable with an electronic pH meter.
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post #8 of 17 (permalink) Old 03-14-2006, 02:06 PM
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Quote:
Originally Posted by guaiac_boy
If you don't account for the other acids present you'll underestimate the amount of CO2 you're putting in.
This has become a pretty hot topic recently. Many people are now questioning the validity of the pH/KH chart. KH test kits also measure other acids in the water as stated by guaiac boy and are creating false high readings in some circumstances. Some are now suggesting ignoring KH and relying on a 1.0 drop in pH instead. I can understand using this method if you are sure you are getting false KH readings, but how do you confirm this?

If I take some tank water and let it aerate for 24 hours or so and then check the pH/KH, I would expect the CO2 level to be somewhere around the 2-5 mg/l level. I assume if the alleged CO2 level was much higher than this it would confirm other acids/bases were present in the water and affecting the KH reading.

Would it be safe to assume an equilibrium level of 2-5 mg/l of degassed tank water, means the pH/KH chart could be used to give a reasonably accurate CO2 level?

Or is it much more complicated than this? Do people have issues with the validity of the chart itself, or just the KH tests?
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post #9 of 17 (permalink) Old 03-14-2006, 03:26 PM
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My tests indicate that water in my house hits about 4 ppm CO2 when it is exposed for 4 hours or longer, but that number could be wrong too. So, I check the CO2 both by the conventional KH/PH method and by checking the decrease in PH for a sample that sits out in the open overnight. If they agree reasonably well then the conventional method works fine. If the conventional method gives a much higher reading, then I assume the method of letting the sample sit overnight will give a more accurate, but still not very accurate reading. Right now, for example, the overnight method says I have 50 ppm, but the conventional method says I have 60 ppm. I'm guessing I really have 30-40 ppm.

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post #10 of 17 (permalink) Old 03-14-2006, 08:35 PM
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Thanks for the response Hoppy.

I was thinking that people who were seeing excessively high CO2 levels with the pH/KH chart, would also see high readings at the lower end of the scale after the tank water was allowed to off gas.

If someone were to use the 1.0 pH drop method and had a KH of 7 with a pH of 6.4, the chart would give them an alleged 83.6 mg/l CO2. If this were allowed to off gas and the pH rise again by 1.0, the CO2 level would be no lower than 8.4 mg/l. This would indicate other acids were being recognized by the KH test and be the reason for the false reading.

After seeing your results though, this appears not to be the case, unless your pH readings are off and causing the issue.

My water always reaches an equilibrium of between 2-4 mg/l and I don't have any issues with the pH/KH chart, so this was all speculation on my part.

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post #11 of 17 (permalink) Old 03-15-2006, 12:21 AM
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The issue isn't really that the KH reading is off. KH test kits work off of an acid titration. Anything in the water capable of bufferning an acid be interpreted as KH. Adding acid (CO2) to the water will drop the pH quickly if the KH is low, and slowly if the KH is high.

In my mind, the presense of other acids in the aquarium water has an even more pronounced affect on the pH/KH/CO2 chart. Decomposing driftwood adds tannic acid. Fish produce a large quantitiy of ammonia over time as anyone who has added tons of fish to a non-planted, non-cycled tank knows. As bacteria convert ammonia/ammonium --> nitrite --> nitrate, a large amount of acid is produced as a byproduct. The exact chemistry is 'out there' if you want to look for it, but the net result is that the hydrogen atoms from NH3 or NH4 are released as N is converted to nitrate (NO3). This can cause your baseline pH readings to be lower than expected, resulting in an overestimation of dissolved CO2.

Does any of this matter? Maybe not, unless a newbie is afraid to add more CO2 since the chart says he/she already has X ppm. It's similar to the WPG thing. One person's 3 WPG is another person's 6 WPG. It's just a lousy unit of light. It just helps to remember that quoting a certain ppm of CO2 isn't as exact as we like to think sometimes.

Last edited by guaiac_boy; 03-15-2006 at 12:48 PM.
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post #12 of 17 (permalink) Old 03-15-2006, 01:22 AM
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Just to complicate an ever more complicated subject: the high CO2 readings you sometimes get and don't believe may actually be close to correct! Not all fish react immediately to over 30 ppm CO2. For example, a couple of nights ago I checked my tank by the "take a sample and wait overnight" method - it said I had about 50 ppm CO2, but the fish seemed to be normal. The conventional KH/PH test said I had around 60+ ppm. I dropped the bubble rate a bit just to be safe. Today I have had 5 fish die and the others all at the top of the tank. So, I turned off the CO2. All day the plants pearled like mad, but the fish have gradually returned to the center of the tank. I now have reset the CO2 needle valve and regulator considerably lower and will see how it goes tomorrow. I had installed a CO2 Mist system Sunday, with a powerhead and the CO2 tube in its inlet, pumping into a long spray bar. It filled the tank with very, very tiny bubbles. I'm assuming that this was a more efficient way to dissolve CO2 into the water plus the absence of the pressure drop across the ceramic disc diffuser affected the operation.

So, maybe it is best to believe some of those high CO2 numbers?

Hoppy
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post #13 of 17 (permalink) Old 03-15-2006, 12:55 PM
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Quote:
Originally Posted by guaiac_boy
The exact chemistry is 'out there' if you want to look for it, but the net result is that the hydrogen atoms from NH3 or NH4 are released as N is converted to nitrate (NO3). This can cause your baseline pH readings to be lower than expected, resulting in an overestimation of dissolved CO2.
guaiac boy,

Thanks for the input. Do you think high levels of these acids would be common in tanks with large weekly water changes? If so, as well as dropping the pH, wouldn't another acid also interfere with the result of a KH test? I thought carbonic acid from CO2 injection was the only acid that didn't interfere with KH.

Going back to my original point. I've been doing a search and have come across an old thread in which Steve Hampton states tank water left to aerate overnight should reach an equilibrium level of 2-5 ppm of ambient Co2. If it doesn't then you have a KH test kit interference. Any thoughts?

Hoppy,

Sorry to hear about your fish losses. I think the debate over actual CO2 levels will continue for a long time. I know many people are now slowly turning up the CO2 until they see fish stress and then backing off a bit. Even this doesn't appear to be totally safe, as some are reporting new fish additions can die quickly.
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post #14 of 17 (permalink) Old 03-15-2006, 01:00 PM
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We'll probably be stuck forever somewhere between using enough to keep the algae away and not so much that it kills the fish. It's kind of like giving rat poision (warfarin) to humans. In high doses massive internal bleeding leads to rapid death. In low doses it is the standard treatment for preventing blood clots and pulmonary embolism. It's prescribed every day all over the world. Its theraputic index (range between useful and toxic) is very small.

Too bad about the fish hoppy. I recently lost some rummies in a simmilar way. Just one more reminder to make changes very slowly. (I'm not saying you didn't - judging from your posts you certainly understand what's going on.) Once the CO2 is high enough to promote good plant growth and keep algae away there is no reason to push it.
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post #15 of 17 (permalink) Old 03-15-2006, 01:14 PM
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Quote:
Originally Posted by Iggy
guaiac boy,

Thanks for the input. Do you think high levels of these acids would be common in tanks with large weekly water changes? If so, as well as dropping the pH, wouldn't another acid also interfere with the result of a KH test? I thought carbonic acid from CO2 injection was the only acid that didn't interfere with KH.

Going back to my original point. I've been doing a search and have come across an old thread in which Steve Hampton states tank water left to aerate overnight should reach an equilibrium level of 2-5 ppm of ambient Co2. If it doesn't then you have a KH test kit interference. Any thoughts?
I think large weekly WC's should greatly diminish the presense and effect of acid buildup from organic breakdown products. Another acid present in the solution would have consumed some of the buffer that was initially there, but the test should accurately show the buffering capacity remaining. Acidity from CO2 addition is a special case since CO2 is in equilibrium with its own buffer.

CO2 + H2O <--> H2CO3 (carbonic acid) <--> H+ (acid) + HCO3- (bicarbonate)

I'm still trying to get my head around the acid/base chemistry involved here.

As far as actual equilibrium CO2 concentrations, the equations state that it should be about 0.5 ppm for distilled water at room temp. People in the aquarium world have been quoting 3-4 ppm for years. I'm not sure where the difference comes from, but I'm guessing they're reporting what they read from the KH/pH/CO2 chart from observations in their own aquarium water when allowed to come to equilibirum. They may be thrown off by the buffering capacity of their water or by acids they didn't account for.
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