During photosynthesis plants combine carbon dioxide with water to produce sugar and oxygen. Land plants absorb the carbon dioxide they need from the air, while aquatic plants use carbon dioxide that is dissolved in the water. The amount of carbon dioxide in aquarium water tends to be quite low, and this places a restraint on the rate at which plants can photosynthesis. By adding extra carbon dioxide aquarists can reduce this restriction and thereby allow plants to photosynthesise more quickly. The quicker plants photosynthesise, the more sugar and oxygen they produce, and the faster they will grow.
Although carbon dioxide fertilisation will benefit all planted aquaria to some degree, the value of carbon dioxide fertilisation is often misunderstood. Furthermore, the methods used to provide carbon dioxide fertilisation are varied, expensive, and in some cases potentially dangerous if not used properly. Finally, carbon dioxide fertilisation isn't a magic bullet that cures all ills; there are many situations where other problems, such as poor lighting, are restricting plant growth.
Is lack of carbon dioxide holding back plant growth in your aquarium?
Let's start with the basics. Carbon dioxide fertilisation is not the most common limiting factor on aquarium plant growth. The two most critical ones are these: whether or not the plant is a true aquatic anyway; and whether there is sufficiently intense lighting for aquatic plants to grow.
Numerous non-aquatic plants are traded, and these are most often bought by newcomers to the hobby. Commonly traded non-aquatic species include Japanese rush (Acorus gramineus); Malayan swordplant (Aglaonema simplex); wheat plant or Siam lily (Chlorophytum bichetii); umbrella plant or aquatic palm (Cyperus alternifolius); dragon and bamboo plants (various Dracaena species); hairgrass (Eleocharis spp.); nerve plant (Fittonia verschaffeltii); purple waffle (Hemigraphis exotica); mondo grass (Ophiopogon japonicus); aluminium plant (Pilea cadierei); umbrella or peacock fern (Selaginella willdenovii); and aquatic or Borneo fern (Trichomanes javanicum). It doesn't matter how well you maintain your aquaria, these non-aquatic plants simply won't live for long when submerged underwater.
As for light intensity, it's a sad fact that the built-in lighting systems found on many all-in-one aquaria are of little to no usefulness as far as growing plants goes. While the watts-per-gallon rule can be criticised on many levels, there is an essential truth to the fact you need to provide above a certain intensity of light for plants to grow. As a very rough estimate, you'll need about 1-1.5 watts/gallon for shade-tolerant plants like Java fern and Anubias species, while most other plants will require anything from 2-4 watts/gallon depending on how 'light hungry' they are. Bushy pink and light green stem plants such as Hygrophila and Rotala tend to need a lot of light, while dark green to reddish-brown rosette plants like Cryptocoryne species will be happy enough with a bit less light.
There are other important factors as well. The depth and quality of the substrate can make a difference. Epiphytic plants (such as Anubias spp. and Java fern) and floating plants (such as Indian Fern and hornwort) don't care about the substrate at all, but plants with roots will at least want sufficient substrate depth to anchor themselves down. A nutrient-rich substrate can be very helpful, though to a very large extent plain washed gravel is fine provided rooted plants receive sufficient mineral nutrients via pellet or liquid fertilisers.
Like fish, plant species have preferences in terms of water chemistry, water current, and water temperature. Vallisneria species for example usually do best in hard water, while the orchid lily Barclaya longifolia requires soft water to do well. Temperate zone plants like foxtails (Myriophyllum spp.) and pondweed (Egeria spp.) are difficult to maintain in tropical tanks, while tropical plants such as crypts (Cryptocoryne spp.) won't last long in a coldwater tank. Some plants like strong water currents, Anubias and Vallisneria for example, while others get damaged or break up if exposed to buffeting currents, and large or herbivorous fish can be just as damaging if kept with the wrong sorts of plants. Severums and silver dollars will ignore Java ferns, but floating Indian fern is nothing more than a salad bar to these herbivorous fish.
In short, if you're having problems growing plants, start by analysing the conditions in the aquarium and the plants you're trying to grow. There's a good chance that you simply have the wrong plants of the set of conditions present in your aquarium. Assuming that your aquarium is offering at least moderately good lighting, there's every chance that swapping the failing plant species for ones better suited to your aquarium will lead to better results.
When is carbon dioxide fertilisation appropriate?
Carbon dioxide fertilisation is necessary when all other factors in the aquarium are favourable but you're still not getting good plant growth. In other words, you're providing enough light for even the most demanding plant species in the tank, the substrate is adequate for the types of plants being kept, and you are providing regular doses of the mineral nutrients your plants need to stay healthy. Water chemistry and temperature are appropriate, and the water current is within the tolerances of the plants being grown.
However, carbon dioxide fertilisation cannot be used in all situations. In tanks with strong water currents, especially turbulent currents that churn air and water together, any carbon dioxide added to the water will be quickly released into the atmosphere. Using carbon dioxide fertilisation in hillstream biotope tanks for example will probably be pointless. There is also a connection between pH and carbon dioxide fertilisation which we'll review in more depth shortly. But for now the key thing is that in very hard, very alkaline water carbon dioxide fertilisation may not be practical, and plants adapted to such water conditions tend not to be fussed about carbon dioxide fertilisation anyway because they're able to use carbonate hardness as their carbon source if there isn't enough carbon dioxide in the water. So a hard water aquarium set up for Central American livebearers shouldn't need carbon dioxide fertilisation, provided the right sorts of plants are chosen to begin with.
Finally, and this is very important, carbon dioxide fertilisation is expensive, tricky to install and maintain, and if used incorrectly can stress or even kill your aquarium fish. If you're not completely comfortable that your fishkeeping skills are reasonably advanced, it's best not to bother with carbon dioxide fertilisation, at least not for a while. It's perfectly possible to create a lovely planted aquarium without carbon dioxide fertilisation, and you'll be better off spending time learning about water quality, biological filtration, water chemistry, and all the other more pressing issues that define successful fishkeeping.
Yeast versus pressurised carbon dioxide systems
There are two basic methods of carbon dioxide fertilisation. One method relies upon yeast fermentation to produce the carbon dioxide, the other uses pressurised cylinders that can bought from hardware stores, brewer supply stores, scientific supply houses and the like. The chief advantage of the yeast fermentation method is that it is relatively cheap to set up, but its shortcomings are several. Yeast fermentation is inconsistent, and getting the the right amount of carbon dioxide being produced on a continual basis is difficult. The lack of commercially manufactured gear means that a certain amount of DIY skill is required to cobble together the necessary components. Finally, the amount of carbon dioxide produced by a yeast culture is relatively small, so for large planted aquarium several vats of yeast need to be kept going, and that quickly makes the whole project cumbersome and hard to maintain.
Pressurised carbon dioxide is the approach recommended here. Although initial set-up costs are substantial, replenishing spent carbon dioxide bottles is cheap and easy to do, and there is a wide variety of hardware available that automates the system, making use and maintenance much simpler over the long term.
At its simplest, a pressurised carbon dioxide system includes the following components: (1) a cylinder of carbon dioxide; (2) a two-gauge regulator and needle valve that sets the amount of carbon dioxide released by the cylinder; (3) a timer that stops the flow of carbon dioxide during the night; and (4) a diffuser/bubble counter that maximises the contact between the carbon dioxide and the water in your aquarium and allows you to determine how much carbon dioxide is being released to the tank per second or minute. These will all be connected with standard airline hosing and carbon dioxide-resistant connectors (typically brass or steel rather than plastic). Some aquarists mix-and-match their kit using some aquarium-specific components together with generic, off-the-shelf ones. That's fine, so long as all the components can be connected together, so make sure you have the right adapters to connect each piece with the airline hose. Other aquarists prefer to buy equipment from a single manufacturer, in which case this shouldn't be an issue. As well as these components, a carbon dioxide test kit is extremely useful.
The pressurised cylinder
Some carbon dioxide fertilisation kits often come with a can of carbon dioxide that looks like a normal aerosol can. These are disposal items, and over the long term work out very expensive. This is particularly the case with the low-end kits, but also holds for systems like the Tetra Plant Optimat system, which uses non-refillable CO2-Depot cylinders. Other companies may not use aerosol cans, but the carbon dioxide used in their systems come from non-replaceable containers anyway, as is the case with the Hydro CO2 Pro System that uses disposable cylinders.
Generic carbon dioxide cylinders are much more economical because these can be refilled as necessary, reducing costs in the long run. In the US the standard carbon dioxide cylinders are 2.5, 5, 10 and 20 pounds in weight. Refilling these cylinders costs the same per unit of carbon dioxide, but a bigger cylinder will need to be refilled less frequently, so it's a good idea to buy the biggest cylinder you can easily carry. Even if your carbon dioxide fertilisation kit initially came with disposable cylinders or aerosol cans, it may well be possible to replace those carbon dioxide sources with a generic carbon dioxide cylinder once it runs out.
The regulator and needle valv e
Regulators are essentially taps (faucets) that let carbon dioxide out of the cylinder at a particular rate. We'll look at that rate in due course, but for now the key thing is to choose a regulator with not one but two gauges (dials) on it. One of these gauges shows the pressure of the carbon dioxide in the cylinder (this is the high-pressure dial) and the other shows the pressure of the carbon dioxide coming down the hose (this is the low-pressure or delivery dial). The first dial shows you how much gas there is in the cylinder, and the second one how much carbon dioxide is being sent to the aquarium.
You can buy regulators with just a single gauge; specifically, the high-pressure dial telling you about the pressure of gas in the cylinder. Such regulators can work fine, but they're less easy to use safely because you won't have as much information to work with. So they're best avoided, and it's much better you stick with a regulator that has two gauges.
Although it is possible to use a regulator to set the amount of carbon dioxide escaping from the cylinder to just the right amount, it's often very difficult to do so. The problem is that regulators just don't provide the fine degree of control aquarists need. So it's a very good idea to install a needle valve after the regulator. With a needle valve installed, it's possible to dial down the flow rate of gas to just a few tiny bubbles per minute, which is what you want. The smaller the bubbles, the more efficiently carbon dioxide will dissolve into the water, improving results and lowering costs.
The timer (sometimes called a solenoid)
Plants don't need carbon dioxide at night, so you may as well stop the release of carbon dioxide from the carbon dioxide cylinder during the hours of darkness. You could do that manually using the regulator, but it's a lot easier to use a timer that automatically shuts down the carbon dioxide flow at preset times. Timers are typically connected between the regulator and the needle valve or between the needle valve and the diffuser.
The diffuser and bubble counter
The final piece of kit mixes the carbon dioxide with the water. Because carbon dioxide is less dense than water it rises, so the basic operation of diffusers is to delay that rise and thereby increase the length of time for the carbon dioxide to dissolve into the water. One of the most popular such devices is the bubble ladder, a clear plastic structure containing zig-zagging tubes that force the bubble to wind its way upwards through the water, all the time yielding carbon dioxide to the water. There are some other designs, but the bubble ladder probably has the best balance of cost against effectiveness. The very basic bell chamber diffusers are cheap but don't work particularly well, while the diffusers using ceramic discs (such as those from Aqua Design Amano) probably work better than bubble ladders but also cost a lot more.
It's a good idea to place a bubble counter just ahead of the diffuser so that the rate at which carbon dioxide is being introduced into the aquarium can be measured. The bubble ladder is effectively a bubble counter as well, because you can count the number of bubbles reaching the top of the ladder per minute.
How much carbon dioxide do you need?
This is where things start to get complicated. In terms of how much carbon dioxide comes out of the cylinder, the standard working pressure is 5-20 pounds per square inch (psi). But the actual amount of carbon dioxide you need to go into the aquarium is a tricky thing to determine. One approach is simple trial and error. Use the needle valve to set the carbon dioxide flow rate to a very low rate, one bubble per second, and see what happens. If after a 3-4 hours there are shiny oxygen bubbles on the leaves of your fast-growing plant species, then you're providing enough carbon dioxide for a good rate of photosynthesis. The appearance of these bubbles is called 'pearling' and indicates the plants are producing lots of oxygen. If your plants aren't pearling, then raise the rate of carbon dioxide provision a notch higher, to two bubbles per second. Wait another few hours, and see what happens. Do this across a few days, and eventually you'll hit the optimal number of bubbles per second.
For an average planted aquarium a carbon dioxide concentration between 10-20 mg/l (or ppt) will generally work best. You can buy carbon dioxide test kits to measure the amount of carbon dioxide in the water. But there's also a useful relationship between pH, carbonate hardness, and the concentration of carbon dioxide, and this can be used to calculate the carbon dioxide concentration. Carbon dioxide (in mg/l) = 3 x carbonate hardness (in degrees KH) x10(7-pH). So for example if the carbonate hardness is 5 Ë°KH and the pH is 7.6, then the amount of carbon dioxide in the water will be 3.78 mg/l.
CO2 (mg/l) = 3 x 5 x10(7-7.6)
CO2 (mg/l) = 3 x 5 x10(-0.6)
CO2 (mg/l) = 3 x 1.26
CO2 (mg/l) = 3.78
Because carbon dioxide becomes toxic to fish above a certain level (typically 25 mg/l) it's a a good idea to start at 10 mg/l and gradually work your way upwards only if you find you need to. Even relatively small amounts of carbon dioxide fertilisation can dramatically improve the growth of fast-growing plant species, and obviously the less carbon dioxide you use, the longer your carbon dioxide cylinder will last between refills.
Potential risks and problems
In the wild there's a day/night cycle between pH and the amount of carbon dioxide in the water. When carbon dioxide dissolves in water it produces carbonic acid, a weak acid. Aquatic plants respire 24-hours a day, but during the day the carbon dioxide they produce this way is offset by the carbon dioxide they absorb during photosynthesis. The net result is that the amount of carbon dioxide goes down during the day, so there's less carbonic acid in the water, and the pH goes up. During the night there's no photosynthesis, so the carbon dioxide aquatic plants release forms carbonic acid in the water, lowering the pH. Big bodies of water like lakes and rivers experience negligible pH changes, but the pH levels of small ponds can vary considerably, from around 7.0 during the night to over 9.0 during a hot, bright summer day. While a similar pH cycle may be seen in an aquarium, we don't want to expose our fish to anything as extreme as this. When fish are exposed to stressful pH changes they often become nervous or exhibit exaggerated gill movements. Watch out for such things when setting up and running in your carbon dioxide fertilisation system.
Carbon dioxide itself is toxic to fish above around 25 mg/l, so it's very important not to use carbon dioxide carelessly. There's some variation among species, air-breathers like labyrinth fish being peculiarly well-adapted to stagnant water conditions, while species from fast-flowing streams, like swordtails and minnows, will become stressed by high carbon dioxide levels much more quickly.
There are various ways to automate carbon dioxide fertilisation systems. One of the most popular relies on pH. As stated above, the concentration of carbon dioxide is related to the carbonate hardness and pH of the water. Given the carbonate hardness is approximately constant, a certain amount of carbon dioxide will lower the pH to a certain level. If we have an optimal concentration of carbon dioxide, for any given carbonate hardness there will be resulting pH value, typically around 7.0. If the pH level goes above that value, then more carbon dioxide needs to be added; once the pH reaches that value, then carbon dioxide does not need to be added any more. Using an electronic pH meter these automated carbon dioxide fertilisation systems top up the carbon dioxide at just the right amount to maintain the correct concentration for good plant growth. While expensive, these devices take a lot of the hard work out of carbon dioxide fertilisation.
The use of carbon dioxide to improve plant growth has been promoted by numerous companies in recent years, in particular Dupla in Europe during the 1980s and more recently by Aqua Design Amano in Japan and the US. Certainly it is true that when carbon dioxide is used properly the results can be amazing, and it's very difficult to get the same diversity of healthy, fast-growing aquarium plants without using carbon dioxide.
But with that said, carbon dioxide is strictly optional, and aquarists with plant problems often wrongly assume that the lack of carbon dioxide is the reason for their failure. Almost always, it's something else, whether light intensity, substrate quality, lack of mineral nutrients, or the wrong environmental conditions. Because careless use of carbon dioxide can cause all sorts of problems for your fish, carbon dioxide fertilisation is not something to approach on a whim. It's important to take some time reading around the subject before spending what will likely be $100 or more on the necessary hardware. Practically every modern book on aquarium plants contains a section on carbon dioxide fertilisation, and finding one to suit your budget shouldn't be hard. There are also any number of online forums dedicated to 'aquarium gardening' and these will provide plenty of inspiration, advice, and first-hand experiences.
K. Horst & H. Kipper (1986) The Optimum Aquarium
The Krib - CO2 & Water Hardness: http://www.thekrib.com/Plants/CO2/
CO2 fertilisation booklet from JBL: http://www.jbl.de/download/co2/JBL_CO2_en.pdf