The last major factor in a planted aquarium is carbon dioxide (CO2). In a planted aquarium, typically 30 ppm of CO2 are desirable (see below for measuring CO2).
Carbon is an important element for plants; they require this element in order to grow and propagate (i.e. all organisms on Earth are carbon based!).
Terrestrial plants do not require carbon dioxide, as they are able to meet their carbon needs from the atmospheric air. However in the aquatic environment, levels of CO2 are typically 4 ppm (parts per million), and are insufficient for plant growth in the long run. Recall that in higher light tanks, plants are being driven harder, and as such, carbon dioxide (as well as nutrients, see above) become an essential part of the planted aquarium "triangle".
Sources of carbon can be introduced into the planted tank in 2 ways:
1) Seachem Flourish Excel
Using this method, a commercially available chemical (polycycloglutaracetal) is added to the tank as an alternative source of carbon. Note that this is not
carbon dioxide, and is only an alternative to carbon dioxide. It is not as effective as carbon dioxide itself, but can work. This method can be used in smaller tanks, but quickly becomes cost prohibitive in larger tanks.
2) Directly injecting CO2
Directly injecting CO2 is the best method of introducing a carbon source into the planted aquarium. One can accomplish this in one of two ways, each has its advantages and disadvantages.
a) DIY (do it yourself) CO2
Using this method, sugar and yeast are mixed into a container. As the yeast begins to feed on the sugar, carbon dioxide is produced as a byproduct of fermentation. This carbon dioxide can be fed into the water, and provides a source of carbon for plants.
- low initial cost
- effective for smaller tanks
- relatively easy to setup (i.e. not much technical expertise required)
- expensive in the long run
- becomes difficult to maintain in tanks larger than 30 gallons (i.e. more than one bottle of DIY CO2 may be required)
- inconsistent production of CO2
An excellent resource for DIY CO2 can be found here:
b) Pressurized CO2
The second method of introducing carbon dioxide into your tank, this method is preferred for larger tanks where DIY CO2 may not be suitable/too much of a hassle. In this method, a pressurized tank containing CO2 is injected into the aquarium, much like in DIY CO2. The following advantages and disadvantages can be said for pressurized CO2:
- Consistent CO2 flow
- Cheaper in the long run
- Requires less maintenance than DIY CO2
- high initial setup cost (i.e. a tank, regulator, needle valve can run upwards of $150; there are also some optional pieces of equipment such as a solenoid, bubble counter, etc)
- some technical expertise is required (i.e. knowledge of how a regulator works)
When introducing CO2 into the planted aquarium, it is also better to fully dissolve the CO2 than to let it bubble off. Dissolving the CO2 ensures that it is available to plants, and not simply out gassed into the atmosphere. Several methods of effectively getting CO2 into the water column exist.
1) Bell Diffuser: A passive method of CO2 diffusion, this relies on the assumption that CO2 will dissolve into the water column faster than the CO2 is produced (not likely). Not an effective method of introducing CO2 into the aquarium.
2) Feeding the CO2 tube into a filter intake: Slightly more efficient, this method allows the CO2 bubbles to be fed into the intake of a filter, allowing the bubbles to be chopped up by the filter impeller. Be warned that this method is said to shorten the lifespan of the filter impeller.
3) Commercially available "bubble ladders": Hagen makes a product that is known as "bubble ladder". This product allows CO2 bubbles to travel a long a track, allowing the CO2 more time to dissolve into the water column. The ladder is quite large and bulky (in my opinion), and some people may find it aesthetically unpleasing.
4) Ceramic disc diffuser: Typically a glass diffuser that contains a ceramic disc with miniature pores. These diffusers were first made by ADA (Aqua Design Amano). Such diffusers rely on the small pores on the ceramic disc to adequately create mini-CO2 bubbles, vastly increasing the rate of CO2 dissolution in water.
5) Inline CO2 reactor: Most arguably the best method of CO2 dissolution, the inline CO2 reactor is inline with a (canister) filter output. Using this method, the CO2 is very effectively dissolved.