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The tank is not planted, water changes vary but I try to do at least 5 -> 10% once every couple of weeks. The tank has been established for around 6-7 months, but was started with gravel from an older tank, and the nitrates tend to be 5 to 10 ppm. There is not much surface agitation.

Hehe.. Well, I live in the middle of a city known for its rather exacting water quality standards, so I doubt there's anything in my tap water thats throwing it off, but it could be one of the stones or maybe the gravel.

I did end up reconfiguring my filter to provide surface agitation, though, and the pH shot back up to 7.0 within days, so who knows :-P This is all far too chemical for a bio student such as myself :-)

Sham is right, there must be other chemicals involved.

While the relationship between KH, pH, and CO2 levels are generally held as gospel in the aquarium community, they aren't fundamentally true. There are quite a few other variables. Any acid - phosphoric, nitric, or even nucleic - will lower pH WITHOUT increasing CO2.

There are also plenty of Alkaline buffers that are not detected by most KH test kits.

pH test kits aren't all equal either, but few of them are bad. And pH test kits are the only one's in the hobby that are NOT likely to be fooled by common organic chemicals.


The reason the KH-pH-CO2 table is useful is that most people don't have a significant amount of these "phantom" buffers in their water supply. But they COULD be present in anyone's water supply - especially in well water or rural areas.

With no CO2 injection and decent surface agitation, I agree with trystianity that your CO2 level could not be higher than 5 ppm.

And since you've said you're not keeping plants, I wouldn't worry about calculating CO2. Just keep measuring pH and KH.


I talk to much.

I agree that its certainly possible... But a 0.5 unit pH rise based on nothing more than surface agitation would suggest to me that gas exchange was the problem.. Altho I could be wrong :-)

Even without surface movement some co2 is going to degas into the air. The water can't hold that much which is why you have to constantly inject co2 to maintain the levels. I do think it can reach higher than 5ppm though but not 20+. I think your also dealing with some other buffers or chemicals that are throwing the readings off by a bit.

Well, although it is always possible that the test kits aren't reading correctly, it does seem somewhat unlikely that a consistently low pH reading would suddenly come right after some surface agitation is introduced... Besides, I see no reason why CO2 levels couldn't increase in a heavily stocked tank... Diffusion is a finite process.

It is impossible to get that much CO2 without adding it with CO2 injection. Sorry, impossible. Unless you add CO2 to the tank, you should only be sitting very close to equilibrium with the air, usually about 3 ppm CO2 or so. Most excess CO2 will just gas out and be replaced by mostly oxygen and nitrogen. Even with no surface agitation at all and a very heavily stocked tank I would be surprised if you managed to go over about 5 ppm CO2. There are other acids that will lower pH and render the ph/KH/CO2 relationship invalid. Things like PO4 (phosphate especially from chemical buffers), humic/tannic acids, etc. Do you use any chemical "buffers" to alter the pH of your water and/or have a lot of leeching driftwood?

Poor maintenance and an old tank will cause pH to crash in a non planted tank because of an increase in acidic chemicals, such as PO4. Your problem is not actually CO2.

So you either have bad test kits (possible) or your pH reading is being affected by other acids in the tank water. If it were possible to get that much CO2 without injecting it, all of us plant people would have much easier (and cheaper) lives.

Well, the nitrates are at about 5ppm, so I don't think thats it.

Also, I reconfigured the filter to provide some surface agitation, and within 3 days, readings had gone from 30mg/L CO2 with pH 6.5 to 9mg/L with pH 7.0, so I think that's fixed it (No water changes involved).

mg/lt = ppm.

Something up somewhere. I would have thought it's practically impossible to get 27ppm of Co2 with some form of supplementation. It's damn difficult enough even with Co2 injection.

Either KH or pH measurements could be off. Dodgy test kits etc.

KH can increase if there's items in the tanks that leech carbonates i.e. certain types of rock and substrate.

---

Some days you're the pigeon and some days you're the statue.

Remember that age and treachery will always triumph over youth and ability.

I would think old tank syndrome would be accompanied by high nitrates. If there is enough organic material being left in the tank to put off that much co2 and no live plants the nitrates should be quite high. Same with if the tank was heavily stocked. Increasing surface agitation would definitely help dissipate some of the co2 and whether the problem is really old tank syndrome or not an increase in water changes would probably be good. 10% every few weeks is low for a nonplanted tank unless it's very lightly stocked. If the co2 level does not go down with increased surface movement and more water changes/gravel vacs then I would guess the co2 calculations are off for one reason or another.

Thanks :-)

I think I'll need a bigger bucket! :-D

Super... you are not changing water often enough to prevent old tank syndrome! Increase your water changes to 20% each week and your problems should be minimized!

__________
"To steal ideas from one person is plagiarism; to steal from many is research."
researched from Steven Wright

Hi,

My aquarium monitoring program automatically calculates CO2 from pH/kH readings.. Lately, I've been getting around 27 mg/litre, which has me a little worried since its usually 20 or less.

What's an acceptable level in mg/l for a tropical freshwater aquarium?

We never have discussed your maintenance schedule. If you are currently changing 10% of your water each week, I would increase the water changes to 20% twice a week. I would combine the water changes with vvigorous vacuuming of the substrate. It will substantially reduce your nitrates and any contaminants in your tank that would add to the CO2 increase.

__________
"To steal ideas from one person is plagiarism; to steal from many is research."
researched from Steven Wright

Regardless what measure you use that seems a rather high amount for a tank without co2 injection. How much surface movement do you have and how long has the tank been setup? Maybe I missed it but is this a planted tank?

Also read through this site especially The pH-KH-CO2 Relationship section.
http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm

Last edited by sham at 09-Sep-2005 22:21

I know, but in this case I'm still suspicious... PPM is a quantitative measurement, while mg is a qualitative measurement... It'd be like trying to compare car lengths/hour and kph - it would all depend on the length of the car. Either way, I'll assume that its close enough for now given how small the concentrations are.

What do you recommend I try?

Gee,

We oftentimes mix measurements whether it is speed and time (KPH) or weight and volume mg/l. In the instance of dilute aqueous solutions 1mg/l is the same as 1 ppm (part per million).

There are several sources of CO2 within the aquarium. One fifth of the CO2 assimilated by the plants is actually returned to the water. The fish also add to the CO2 levels with their respiration. The bacterial decay of both dead plant material and driftwood decorations in the tank produce CO2. But the largest contributor to CO2 levels in the aquarium is the biofilter. In fact, a fully cycled biofilter will produce 1 gram of CO2 within 24 hours for every gram of dry food added to the tank.

from Cichlid-forum

The bio-filter is an established aquarium is the source of "Old tank" syndrome, a characteristic of which is declining pH as decay increases CO2.

Ingo is close on his perception of the relationship between pH, CO2 and KH. Essentially, KH is the fixed component of the relationship, pH is the measure or the indicator of the level of CO2 in the aquarium. The variable in this relationship is then CO2 a fact that is indicated by the change in the indicator, pH.

Last edited by Bob Wesolowski at 09-Sep-2005 14:24

__________
"To steal ideas from one person is plagiarism; to steal from many is research."
researched from Steven Wright

Bob - thankyou for noticing - so many people don't :-P You will also note that our flag (which of course I think is better ;-)) has 5 pointed stars that are red with a white outline, while our Australian neighbours have mostly 7 pointed stars that are solid white :-D

With regards the spelling, call it a dialectal difference :-) We also use spellings such as colour and analyse as opposed to color and analyze. We are an odd bunch. :-D

I don't inject CO2 (nor for that matter have I ever seen a CO2 injector for sale here in New Zealand), but then again, I do not keep live plants in this aquarium, so it hasn't concerned me up until now.

I know we're talking water, but mg is a unit of weight, and one molecule of carbon dioxide weighs over twice as much as one molecule of water (CO2 = 44 g/mol, H2O = 18 g/mol, one mole being 6.02 x 10^23 particles). If one in every million molecules in the solution was carbon dioxide, I don't think that would mean that one in every million mg was carbon dioxide... As an example, if I had 1mg of water, there would be 3.34 x 10^19 molecules of water... If I had 1mg of carbon dioxide, there would be only 1.37 x 10^19... <thinks> Of course, having said that, I still don't know what the conversion rate should be.

Ingo - I have often wondered that myself (how the tank kH can be higher than the tap pH) - it seems to be linked with certain kinds of filter media, although I've never understood how. Using Ferric Oxide Hydroxide (which to my knowledge does not contain carbonate) has increased it in the past. I theorise that perhaps these media types influence pH, rather than kH, and there is a play off effect?

The only recent change (since adding this new filter) has been a fall in pH. Previously, pH would read at, or extremely close to neutral.

The old filters that I have used were either submersible power filters and/or Hang-On-Back running either carbon and/or Ferric Oxide Hydroxide... The new filter is a much larger and more powerful Canister, running carbon, ferric oxide hydroxide and four resins as chemical media.



Last edited by geesloper at 07-Sep-2005 19:27

I want to throw in a question here:

It is my understanding that ph and KH influence the CO2, but also that having a certain ph and adding CO2 will NOT alter the KH. For example, having KH of 2 and a ph of 7.2, adding CO2 to the water will NOT raise the KH but lower the ph.

Is that wrong?

I believe it isn’t, so how does your KH go from 2 (tab) to 3.1?

Sorry I didn’t catch this earlier.

Ingo

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Proud Member of the New Jersey Aquatic Gardeners Club

Sorry, Gee. The small flag size shouldn't have prevented me from noting the loss of the Commonwealth Star or Epsilon Crux in the Southern Cross. My apologies.

By the way, we are talking water as the measure or 27 mg/l is milligrams per litre of watre. Now, why do you spell liter as litre after all you don't spell water or watre.

If you can double check your readings with someone else's test kit and correspond your readings to your water source so that you have a base line. That is, if your tap water is 7.2 pH with a KH of 3.1, then you can see the impact of CO2 on your pH.

I assume (hope) you are injecting CO2. We can assume that your plants are unable to fully utilize the level of CO2 that you have injected into your system. Belying my previous post, see this link from Tropica:

http://www.tropica.dk/article.asp?type=aquaristic&id=417

At your pH, optimum CO2 should be 21 mg/L. So, dial down your injection or degas the CO2 by increasing surface agitation or plant more plants.

__________
"To steal ideas from one person is plagiarism; to steal from many is research."
researched from Steven Wright