Carbon Dioxide Enrichment
To photosynthesize plants need light, CO2 and nutrients. If either of these is deficient the plant will not grow and photosynthesize properly. If there is not enough light the plants will not be able to use the CO2 in the water or visa versa.
Carbonate Hardness and General Hardness
The two different types of hardness KH and GH are often confused. Carbonate hardness is the concentration of carbonates (CaCO3) in the water and general hardness refers to the dissolved minerals like calcium and magnesium in the water.
When talking about CO2, KH is the hardness that is important. CO2 is not only important in the aquarium because plants need it to photosynthesize but also because of how it affects the pH of the water. Dissolved carbonates will raise both the KH and pH of the water and CO2 will lower the pH by producing carbonic acid.
This relationship can be seen in the table below.
CO2 and Carbonate hardness
table |
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| KH/pH | To much | Optimal | To little | ||||||||
| 6,0 | 6,2 | 6,4 | 6,6 | 6,8 | 7,0 | 7,2 | 7,4 | 7,6 | 7,8 | 8,0 | |
| 0,5 | 15 | 9,3 | 5,9 | 3,7 | 2,4 | 1,5 | 0,93 | 0,59 | 0,37 | 0,24 | 0,15 |
| 1,0 | 30 | 18,6 | 11,8 | 7,4 | 4,7 | 3,0 | 1,86 | 1,18 | 0,74 | 0,47 | 0,30 |
| 1,5 | 44 | 28 | 17,6 | 11,1 | 7,0 | 4,4 | 2,8 | 1,76 | 1,11 | 0,70 | 0,44 |
| 2,0 | 59 | 37 | 24 | 14,8 | 9,4 | 5,9 | 3,7 | 2,4 | 1,48 | 0,94 | 0,59 |
| 2,5 | 73 | 46 | 30 | 18,5 | 11,8 | 7,3 | 4,6 | 3,0 | 1,85 | 1,18 | 0,73 |
| 3,0 | 87 | 56 | 35 | 22 | 14 | 8,7 | 5,6 | 3,5 | 2,2 | 1,4 | 0,87 |
| 3,5 | 103 | 65 | 41 | 26 | 16,4 | 10,3 | 6,5 | 4,1 | 2,6 | 1,64 | 1,03 |
| 4,0 | 118 | 75 | 47 | 30 | 18,7 | 11,8 | 7,5 | 4,7 | 3,0 | 1,87 | 1,18 |
| 5,0 | 147 | 93 | 59 | 37 | 23 | 14,7 | 9,3 | 5,9 | 3,7 | 2,3 | 1,47 |
| 6,0 | 177 | 112 | 71 | 45 | 28 | 17,7 | 11,2 | 7,1 | 4,5 | 2,8 | 1,77 |
| 8,0 | 240 | 149 | 94 | 59 | 37 | 24 | 14,9 | 9,4 | 5,9 | 3,7 | 2,3 |
| 10 | 300 | 186 | 118 | 74 | 47 | 30 | 18,6 | 11,8 | 7,4 | 4,7 | 3,0 |
| 15 | 440 | 280 | 176 | 111 | 70 | 44 | 28 | 17,6 | 11,1 | 7,0 | 4,4 |
| 20 | 590 | 370 | 240 | 148 | 94 | 59 | 37 | 24 | 14,8 | 9,4 | 5,9 |
CO2 in Milligrams per liter |
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Plants generally need at least 5mg/l CO2 to photosynthesize and most aquatic plants grown for the aquarium need a slightly acid water (this also helps with nutrient absorption and helps keep nutrients like Fe available). Fish generally don't like more than 20mg/l CO2 so as a rule of thumb the right amount of CO2 in the water 5-20 mg/l CO2 would be good for a normal aquarium. This would obviously differ with different types of plants and lighting.
There are various testing kits available for pH and hardness. The CO2-KH-pH equilibrium should be quite accurate if your tests are accurate unless you are changing the water allot by adding buffers or acids.
CO2 Generation
The most common method of CO2 generation is the use of a Gas cylinder with a regulator to control the flow. There are many variations on this. Other DIY ways of CO2 generation are the use of yeast and sugar brews which are quite effective and very cheap but more difficult to monitor or get accurate results.
CO2 Reactors
These are to get the CO2 dissolved in the water. There are many diff rent types commercially available as well as DIY types. The general principal is to get the CO2 to have as long a contact as possible with the water to give it time to dissolve. It is done in a few ways some of them quite complex others as simple as an upside down collection bowl or dome under the water that the CO2 will stay in till dissolved. Others use air type stones or power heads that that the gas is bubbled through to try and get a high contact with the water.
