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Ok, based on the calculation table for CO2, your values are right, about 27ppm.

But if you had 20ppm before, something must have been different. Either your ph was higher or your KH lower, otherwise, given that you say you don't add anything, your value would not have been 20ppm.

Is that possible?

What is the difference in your new filter?

Ingo

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Proud Member of the New Jersey Aquatic Gardeners Club

Thanks for the quick responses, guys :-)

To answer your questions as best I can:

1) I thought about mg/litre being 1 ppm, but I'm not 100% sure of that. It is true that in our metric system 1 million mg of water would be 1 litre, but we're not talking water, so I don't know if it follows?

2) The aquarium pH is around 6.5

3) kH is around 3.1 dKH

4) Recent changes are only the addition of a new filter with some new media types

5) I do not chemically modify the kH or pH in any way

6) Yes, it is still using 60W of lighting

7) I'm not actually in Australia, I'm in New Zealand (The flags are subtly different :-D) Our water is actually more alkaline (7.2 at least), with a low kH (often not higher than 2 dkH, frequently lower)

Gee,

It is my understanding that CO2 concentration must exceed 30ppm before becoming dangerous to fish. However, a rapid rise of CO2 to that level will harm the fish at it will be accompanied by a precipitous drop in pH.

From past posts, it appears that water in Oz in frequently acidic. I would take a water sample to your LFS for a retest on pH and KH just to verify the efficacy of your test kit.

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

geesloper,

If I am not mistaken then mg/liter is the equivalent of ppm and as such you should be, generally speaking, still in an acceptable range.

What concerns me a little is how you got to the elevated level of CO2.

What is your PH?
What is your KH?
Did you make any changes to the tank recently, in particular lighting, plants, ferts, water change routine, general maintenance routine, fishload, etc.?
Do you raise your KH with baking soda? If so, could it be you overdosed a little?
I assume we are talking about the Aquarium 1 in your profile. Do you still have only 60W of lighting?

Ingo

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Proud Member of the New Jersey Aquatic Gardeners Club