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Lets see here, CO2 is another often talked about subject. Next to lighting, CO2 will give you a huge increase in plant growth. It is often used in of terrestrial plant greenhouses to give them one more advantage. Unlike in terrestrial gardening our plants are under water, and water is unable hold the same amount of free CO2 as the air. Therefore a small addition of CO2 will make a huge difference in plant growth assuming all the other conditions are satisfied. CO2 is an important part of plant growth and this is why the Aquatic Gardener often adds it as part of the fertilizer plan.

CO2 can be added in a number of different ways, but basically you will need a source, and then the associated plumbing to get this supply into the water. Any way you do this I STRONGLY recommend that before you get started you obtain a KH test kit. There is a two fold reason here. First, you do not want to add CO2 to a tank with a KH below 3°(~35ppm). And secondly, once you start injecting you will need a way to monitor the CO2 levels.

When CO2 is added to water they mix and create carbonic acid. This newly formed acid will immediately lower the tanks pH. So if your KH is not above 3° the pH stability will be so low it can be harmful to the fish. Now knowing that the carbonic acid will depress pH is helpful in monitoring your CO2 concentrations. As with most things in nature there is some kind of order and with this there is a logarithmic relationship between your tanks KH, pH, and CO2 levels. You can calculate your CO2 level by using the pH and KH measurements. I will not make you do that, below you will find a table for different KH and pH combinations. Also I have plotted it into an easy to read chart.

To repeat, CO2 additions depress pH because of the carbonic acid made in the mix. pH and KH have no effect on CO2 it is the other way around.

I myself use CO2 on both of my show tanks. I have tanks without it and let me say there is still room for success, but I do like the results with it and it gives you one more advantage over the algae. Therefore I will highly recommend the use of CO2 so next you ask what will it set you back. There are many different methods and levels for injecting CO2. You can go the very inexpensive route, the DIY Yeast reactor for $5, or you can go as high as the fully automated Dupla setup that will run oh near $1500US. Or you can choose something in between. I bought my own compressed gas CO2 bottle from a gas supplier along with a regulator and a fine control metering valve. My setups cost about $200 each. These setups give me a constant supply of CO2 for about a year and I do nearly nothing to them. Once they are setup and running properlly the only thing I need to watch for is the bottle to run dry.

For a cheaper approach you may chose the Home-brew method. This involves a 2 liter coke bottle, about 2 cups of sugar, a teaspoon or so of yeast, pinch of baking soda and maybe a pinch of flour. Plumb a airline into the bottle and allow the mix to bubble into your power filter. Boom you got explosive plant growth for almost nothing, but you have to remix a batch every couple weeks and keep a close eye on it.

There has been SO MUCH written on CO2 injection methods, I will provide a few pointers or things to consider and then refer you to other links.
Remember with the addition of CO2 you will get a pH drop so be consistent with your input or the fish will get stressed!
Leaks are an enemy. Outside of loosing your precious CO2 it will create an inconsistency that will vary the injection rate and well, the pH. So for the fish benefit you want to avoid these inconsistencies.
There is a limit to the amount, some is good, more is not necessarily better. Fish will be stressed at extreme levels and it is suggested you keep it below 35-40ppm. Plus anything over 20ppm doesn't do that much more for the plants.
CO2 does not displace Oxygen so used in moderation there is no concern there.
There is some danger here, we are talking about pressure. NEVER use a glass bottle for your yeast reactor. If it explodes, well aside from the slimy, yeasty, stinky mess, you will have shards of glass flying through the air. Same with the gas bottles, caution is always advised. If that neck/valve gets knocked off (falls off the tailgate) you will create a missile with no guidance control.
There are numerous ways to mix it into the water.
Inverted bells, powerheads, power and canister filters, home made contraptions and store bought CO2 reactors.
Here is a link to everything you will need. The Krib's CO2 & Water Hardness Section This should be all you need.
The Booth's site has a very good section on CO2. Initially they have a good question and answer section, but then a ton of info.

CO2 concentrations as a function of KH and PH

\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     1       0.6     0.2  
1.0   |  30      19      12       7       5       3       2       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  
                 CO2  milligrams/liter 

The typical CO2 level for an un-injected tank is around 1-3 ppm/mg/l
The desired range for a planted tank is 10-20 ppm
Anything over 35 ppm can be harmful to the fish.

Here is another way of presenting it. I thought this might be helpful too.

The equation I used for the plots is:

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Jeff Dietsch

Last updated 7/23/02 JCD