Carbon Dioxide Part Two: Estimating Concentration

by John Tullock

In Part One last week, I discussed the importance of carbon dioxide (CO2) in the planted aquarium, and described several types of equipment used for adding CO2 to improve plant growth. In this post, I will discuss how CO2 concentration can be estimated when the pH and alkalinity of the aquarium are known.

No method exists that would permit the aquarist to directly measure the carbon dioxide concentration of his aquarium. However, the concentration can be estimated closely enough for our purposes, since we want to maintain about 12 to 20 ppm of CO2 during daylight hours.

Below is a table presenting the carbon dioxide concentration in parts per million as a function of pH and alkalinity. The pH values are listed across the top of the table, with alkalinity values (three equivalent values depending upon units used) down the left side. Thus, for example, at pH = 7.0, the optimum CO2 concentration lies at an alkalinity between 4 and 4.5 dKH.

From the table, it is easy to see that lower pH and alkalinity values allow water to dissolve much more CO2. Most aquarists are already aware that the widest variety of plants grows in soft, slightly acid water.

If you plan to use tap water in your planted tank, test it first. Water of high alkalinity, “hard” water, will make your life harder when you use it for aquatic plants. Instead, use distilled water mixed with your tap water to an alkalinity no higher than 5 dKH. If for some reason you wish to increase alkalinity, numerous products are available for this purpose.

Test kits for both pH and alkalinity are widely available, inexpensive, and easy to use. (Always check any test kit against a known standard solution before using it for aquarium purposes.) Test your aquarium at least once a week, preferably at the same time of day, to make sure your plants are getting enough vital CO2.

NEXT: Installing and Testing Carbon Dioxide Systems

TABLE 1: CARBON DIOXIDE CONCENTRATION AS A FUNCTION OF pH AND ALKALINITY

pH–> 6.4 6.5 6.6 6.7 6.8 6.9 7 7.1 7.2 7.3 7.4
dKH mg/L CaCO3 meq/L
3 54 1.07 43 27 16 11 7 4
3.5 63 1.25 50 31 19 13 8 5
4 71 1.43 57 36 21 14 9 6
4.5 80 1.61 64 40 24 16 10 6
5 89 1.79 71 45 27 18 11 7
5.5 98 1.96 79 49 29 20 12 8
6 107 2.14 86 54 32 21 13 9
6.5 116 2.32 93 58 35 23 15 9
7 125 2.50 100 63 38 25 16 10
7.5 134 2.68 107 67 40 27 17 11
8 143 2.86 114 71 43 29 18 11
8.5 152 3.04 121 76 46 30 19 12
9 161 3.21 129 80 48 32 20 13
9.5 170 3.39 136 85 51 34 21 14
10 179 3.57 143 89 54 36 22 14
10.5 188 3.75 150 94 56 38 23 15
11 196 3.93 157 98 59 39 25 16
11.5 205 4.11 164 103 62 41 26 16
12 214 4.29 171 107 64 43 27 17
12.5 223 4.46 179 112 67 45 28 18
13 232 4.64 186 116 70 46 29 19