Determining CO2 levels by testing the pH and KH of your aquarium

by Owen James

CO2 is vital for plant growth, but too much in the water will kill fish

CO2 is vital for plant growth, but too much in the water will kill your fish

(Photo by: nttrbx)

Carbon dioxide (CO2) is a vital nutrient for plants, but too much is deadly to fish. You should therefore monitor your tank’s CO2 levels if you’re dosing with supplementary carbon dioxide, to ensure you are effectively fertilizing your plants without endangering your fish.

One way to work out the level of CO2 in your aquarium water is to test the pH and the carbonate hardness (KH) levels. These readings can then be used with the table below to determine the CO2 concentration of your water.

How to determine the CO2 level using pH and KH

  • Test for both pH and KH using your aquarium test kits
  • Cross-reference both values in the table below
  • The intersection gives you the CO2 level in mg/liter
-----------------------------------------------------------
 \ pH | 6.0   6.2   6.4   6.6   6.8   7.0   7.2   7.4   8.0
KH\   |
-----------------------------------------------------------
0.5   | 15    9.3   5.9   3.7   2.4   1.5   0.9   0.6   0.2
1.0   | 30    19    12    7     5     3     1.9   1.2   0.3
1.5   | 44    28    18    11    7     4     2.8   1.8   0.4
2.0   | 59    37    24    15    9     6     4     2.4   0.6
2.5   | 73    46    30    19    12    7     5     3     0.7
3.0   | 87    56    35    22    14    9     6     4     0.9
3.5   | 103   65    41    26    16    10    7     4     1.0
4.0   | 118   75    47    30    19    12    6     5     1.2
5.0   | 147   93    59    37    23    15    9     6     1.5
6.0   | 177   112   71    45    28    18    11    7     1.8
8.0   | 240   149   94    59    37    24    15    9     2.4
10    | 300   186   118   74    47    30    19    12    3
15    | 440   280   176   111   70    44    28    18    4
------------------------------------------------------------

Important notes on CO2 levels
The optimal safe C02 level for plant growth is 15-35 mg/l.
CO2 levels above 35 mg/liter are dangerous to fish.
This method is not accurate in tanks using peat or mature aquariums with a lot of organic matter and debris.

The accuracy of this testing method depends especially on an accurate KH reading, so double check your results!

Check out Katy's Tropical Fish Guide for more aquarium info.

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{ 1 comment… read it below or add one }

Mary Leonard 04.07.11 at 3:09 am

Does anyone carbonate their water daily using a Fizz Giz? If you siphon off a 16.9oz (1/2 liter) bottle of water into an appropriately sized soda bottle, at a given temperature (say, 75-d F), a specific amount of co2 can be introduced into the bottle at a specified pressure. The amount can easily be determined in grams. If you know the volume of water in your tank, in liters, you can determine the number of grams of co2 you wish to introduce into the tank and infuse that amount into a common PET soda bottle at a specified pressure. High pressure levels offer the advantage of more thorough and instantaneous infusion. Using this sites PH/KH chart, you can determine the amount used up by the plants EVERY day because you KNOW precisely how much you are putting in. Bubbling co2 through a vertical column of water transfers some co2 to the water as each bubble rises to the top. The larger portion of co2 remains undissolved as it reaches the surface. I would imagine some of it sits there on the surface of the water and is slowly dissolved into the tank water. But it seems likely that most of it dissipates into the air in the room and is lost. I think I would like knowing exactly how much co2 I put into the tank daily and knowing that it is thoroughly dissolved into the water when it is introduced. I’m sure some of the co2 will break the surface of the water and be lost from the system and some will stay. But if the loss at the surface is minimal compared to that comsumed by the plants during photosynthesis, you could pretty much keep a finger on the pulse of your plants’ daily consumption of co2. I think that would be cool to know.

Anyway, just my 2-cents worth. Maybe someone’s already looked into this and has some answers in the form of empirical data to share.

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