Dosing CO2

Posted by on Jul 6, 2011 in All Categories, Aquarium Maintenance | 0 comments

Dosing CO2

For most aquarium plants, the correct CO2 value is 20-30mg/l. This is independent of the Carbonate Hardness (KH) of the water but the resulting pH (acidity) will be influenced by the water’s KH.

pH measuring and controlling devices adjust pH by adding CO2. In planted tanks, the plants’ assimilation during daylight hours will make the water more acidic. Plants deplete the dissolved CO2 (in the form of carbonic acid) quickly as they need it for photosythesis. The consequence of taking away the carbonic acid as an acidifier is that the water becomes more alkaline. pH goes up.

CO2 Cycle Aquarium

CO2 Cycle Aquarium

Once a critical pH values has been reached, the pH meter wil open the magnetic valve and CO2 is replenished: pH goes down.

Which is the correct pH value for the right amount of CO2?

Depending on your tap water, the right pH will be slightly different. “Hard” water with a higher KH has a larger buffering capacity, i.e. it is capable of absorbing more acid. Hence, pH will always be higher in “harder” water for a given amount of dissolved CO2 than in “soft” water.

The following table shows correlation between pH and KH for a given CO2 value. KH is easily measured using drop tests available at aquarium shops. Your local water board probably has some data as well.

  Too much CO2
     CO2 OK
  too little CO2
pH-Value
KH 6.3 6.4 6.5 6.6 6.7 6.8 6.9 7 7.1 7.2 7.3 7.4 7.5
2 32 25 20 16 13 10 8 6 5 4 3 3 2
3 48 38 30 24 19 15 12 10 8 6 5 4 3
4 64 51 40 32 25 20 16 13 10 8 6 5 4
5 80 63 50 40 32 25 20 16 13 10 8 6 5
6 96 76 60 48 38 30 24 19 15 12 10 8 6
7 111 89 70 56 44 35 28 22 18 14 11 9 7
8 127 101 80 64 51 40 32 25 20 16 13 10 8
9 143 114 90 72 57 45 36 29 23 18 14 11 9
10 159 126 100 80 63 50 40 32 25 20 16 13 10
11 175 139 111 88 70 55 44 35 28 22 18 14 11
12 191 152 121 96 76 60 48 38 30 24 19 15 12
13 207 164 131 104 82 65 52 41 33 26 21 16 13
14 223 177 141 112 89 70 56 44 35 28 22 18 14

CO2 (mg/l) in relation to Carbonate Hardness (°d) and pH-Value

Measure your Carbonate Hardness and match with a medium range CO2-content (green band), e.g. 20 mg/l. Then, read off the corresponding pH value and adjust your dosage accordingly.

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