How Accurate are the CO2 Calculators?

[Maverick2007]

I was wondering if anyone could comment on the accuracy of CO2 calculators based on calculating PH and KH. The reason I ask is that based on this method I have an unusual high CO2 measurement which I'm sure is not correct.
Here are the details:
PH - 6.0
KH - 3 degrees


CO2 concentration 90 ppm?!?!?


I'm sure if this was correct my fish would be dead which fortunately is not the case. Also my source of CO2 is DIY with an air stone diffuser.


I look forward to the communities thoughts.


Thanks

[prolude006]

I believe you would need to know your ph and kh before you added co2 to do it that way, plus a few other measurements.

Search here for "co2" there are many threads on it.

[Bk828]

the calculators are very inaccurate since there are other things in the water which will affect the results. Get a drop checker, it will give you more precise reading.

[Maverick2007]

Thanks prolude006 & Bk828 !!
BK828 my understanding of drop checkers is that they are sometimes not very accurate either. From what I have read, the color changes in the drop checker may be affected based on where it is placed (i.e. if its placed close to the source of the CO2 the readings may be higher then in other areas of the aquarium).
I appreciate your thoughts and comments.
Cheers

[Bk828]

If you have flow in the tank then the co2 content throughout the tank should be the same

[KarlTh]

The calculators only work if the only acid is CO2 and the only buffer is bicarbonate. This is seldom the case.

[Maverick2007]

Bk828, when you say "have flow in the tank" are we talking about flow under the surface? I would guess that flow at the surface (i.e. water agitation would promote gaseous depletion of CO2).
Thanks for your suggestion I will certainly look at drop checker....I have read about DIY drop checkers do you know if these are effective?
Thanks again and cheers !!

[Maverick2007]

KarlTH are you referring to the bicarbonate soda that's added to the DIY CO2 or about a bicarbonate buffer?
Thanks

[KarlTh]

I'm referring to the bicarbonate which is part of the bicarbonate system. It doesn't matter where it comes from; it exists in an equilibrium with carbonic acid. It's this equilibrium system which the KH/pH/CO2 tables work on; if there are other acids and/or other buffers, then the tables won't work.

Bicarbonate added to the DIY CO2 is irrelevant; you're not putting that in the tank and we're referring to what's going on in the water.

[oblongshrimp]

If you get a drop checker make sure you buy the standard KH solution. If you don't know the KH of the water your putting into the checker it won't be nearly as accurate, if accurate at all).

[Maverick2007]

KarlTH, please let me know if I'm on the right track in terms of my understanding of the bicarbonate system. In order for plants to grow efficiently they require among other things a carbon source of which CO2 is easiest for the plants to consume. That being said, if the CO2 levels fall off, plants will start to utilize carbon from KH which will lead to an increases in the PH. Therefore in order to prevent fluctuations in PH its necessary to keep levels of CO2 at a constant level or utilize a "bicarbonate buffer" to increase KH.
At present I do not use/dose a bicarbonate buffer would it be wise for me to start considering a DIY CO2 system?
Again thanks again for your insights. Cheers

[Maverick2007]

oblongshrimp thanks for the heads up!! Do you know if commercially purchased drop checkers come prepared with the KH solution included?
Thanks and Cheers

[DAVIDFBT]

^ This one does. (http://cgi.ebay.com/Drop-Checker-CO2-pH-Test-Live-Aquarium-Plant-Diffuser_W0QQitemZ250305624190QQcmdZViewItemQQptZL H_DefaultDomain_0?hash=item250305624190&_trksid=p3286.c0.m14&_trkparms=72%3A1205%7C66%3A2%7C65%3A12%7C39%3A1%7C 240%3A1318)

[KarlTh]

You do not need to add a buffer. The point about buffers is simply that the tables assume that bicarbonate is the only one in use. In some cases that assumption is way off base. Yours may be such a case.

There is such a thing as biogenic decalcification, which is what you're referring to, but adding a buffer doesn't prevent it. Having sufficient CO2 does. Only a few plants can do it anyway.

Plants require CO2. Any other carbon source needs to be turned into CO2 before the plant can use it. Biogenic decalcification is one way in which a few plants can turn another carbon source into CO2.

The bicarbonate system I refer to is an equilibrium which always exists in any water to which CO2 is added:

CO2 + H2O <=> H2CO3 <=> H+ + HCO3-

These three are in an equilibrium. Of these, HCO3- is bicarbonate, i.e. KH. If you add CO2, then the equilibrium will shift to the right, which you will see ends up releasing H+ ions and therefore lowering the pH. Adding bicarbonate will push the equilibrium to the left, causing H+ ions to be taken up, lowering the pH. That, in a nutshell, is the bicarbonate system, and it has a fixed relationship between KH, pH and CO2 concentration.

If there are other acids present, then the overall pH of the water will be lower than the pH predicted purely by the bicarbonate system. If there are other buffers, then the pH will be closer to the equilibrium pH of those buffers than it would otherwise be. Additional acids are usually the issue in aquaria; organic acids from metabolism, tannic acid leached by peat and bogwood, and so on. Hence pH tends to be lower than the bicarbonate system alone would result in, and CO2 calculations come out higher than the real figure.

[Maverick2007]

KarlTH thank you so very much for this very detailed explanation on the bicarbonate system and its relationship to PH, KH and CO2 I'm sure others will benefit as well.
In terms of additional acids you mentioned, I do have grape vine trunk as my "driftwood/bogwood" which I assume could probably leach tannins although I do not visually see any tannins in the water column (i.e. the tint in the color of the water). What in your opinion would be the best way for me to determine my true PH? I suppose I can start by measuring the PH of my water source but then that would not take into account the "acid factor". I hope I'm wrong but I guess the only way to calculate the true PH would be to remove the CO2 from the aquarium for about 24 hours which I'm guessing is probably not a good thing for the plant.
I look forward to your thoughts and or comments.
Thanks and Cheers

[Maverick2007]

DAVIDFBT thanks for the link.
Cheers

[Bk828]

Davids link does give you a good drop checker but the solution there is a ph reagent..
Check www.greenleafaquariums.com they have 4dkh solution for the drop checkers.. or you can make your own

[KarlTh]

You are measuring the true pH. What it doesn't allow you to do is determine the true CO2 concentration. You'll need a drop checker with pure water of known KH (i.e. no acids or buffers other than CO2 and bicarbonate) to do that.

[Star_Rider]

on a side note:
(good description and explanation on the CO2 issues Karl.)

"I do have grape vine trunk as my "driftwood/bogwood" "


word of caution.. grapewood is one of the woods which may deteriorate rather quickly in the tank.

[Maverick2007]

You are measuring the true pH. What it doesn't allow you to do is determine the true CO2 concentration. You'll need a drop checker with pure water of known KH (i.e. no acids or buffers other than CO2 and bicarbonate) to do that.

Thanks for the confirmation looks like I will need to go out and find me a drop checker!!

Cheers

[Maverick2007]

on a side note:
(good description and explanation on the CO2 issues Karl.)

"I do have grape vine trunk as my "driftwood/bogwood" "


word of caution.. grapewood is one of the woods which may deteriorate rather quickly in the tank.


Hey star_rider I've heard the same think and usually from what I've read is due to the trunk not being dried out properly. The one in my tank was dried out for over 2 years. I'll keep a very close eye on it just to make sure no deterioration is happening....Thanks again and Cheers

[Star_Rider]

Hey star_rider I've heard the same think and usually from what I've read is due to the trunk not being dried out properly. The one in my tank was dried out for over 2 years. I'll keep a very close eye on it just to make sure no deterioration is happening....Thanks again and Cheers

actually, it has more to do with the wood structure of grapevine.

loose, cell structure and porous. in essence a low density wood.
but it still should last some time in the tank.