Plants have a few basic requirements that must be met if they are to survive and to thrive. If any one of these factors are in limited supply, the others are effectively wasted (and usually algae takes advantage of these leftovers). In an aquarium the limiting factors are most likely to be (in order): light, carbon dioxide (CO2), micronutrients (trace elements), and macronutrients. Micro and macronutrients are usually supplied in adequate quantities by fish waste. Some CO2 is provided by fish respiration, although with heavy aeration the CO2 level will be the same as in the air. If your lighting is sufficient, at least 0.5 watts per litre, the next most important thing you can do to increase the growth rate of your plants is to inject CO2 (and conversely if your lighting is inadequate, injecting CO2 will not help at all). 
Before proceeding with this or any method of CO2 injection it would be advisable that you read up about both plants and CO2 injection. It is especially important to understand the relationships between CO2 concentration, pH and carbonate hardness, and the ways they can be manipulated. (Carbonate hardness should not be confused with general hardness.)

For those of you who have thought of using carbon dioxide (CO2) injection in your planted aquariums but have been put off by either the cost of available systems or have been unsure what the effects of CO2 injection on plant growth may be, then this article describes an inexpensive and easy to assemble DIY system that uses yeast to produce the CO2. The CO2 injection system consists of two parts: a generator which holds the yeast mixture; and a reactor which ensures the CO2 is efficiently dissolved in the aquarium. The two parts are connected by airline tubing.
 
The Generator
The CO2 generator is simply made from a 2 litre plastic soft-drink bottle. Drill a hole through the lid, force some airline tubing through the hole and seal both sides with silicone sealant. Ensure that this seal is airtight. Alternatively you can drill a hole through a rubber bung (bought from somewhere like Para), force a small-diameter rigid tube (perhaps from an undergravel filter uplift) through, and attach your airline to this. Depending on factors such as the carbonate hardness, number & type of plants, amount of surface agitation, number & size of fish, etc. a 2 litre generator should provide enough CO2 for an aquarium of ( 200 litres. For larger aquariums a larger generator or several generators connected together may be required to produce enough CO2.
 

 
The Reactor
The next step is to build a CO2 reactor. The purpose of a reactor is to aid in dissolving the CO2 in the aquarium water. This can be as simple as using an airstone to produce many small bubbles which will dissolve more quickly than a single large bubble.

Another method is to invert a container, place it in your aquarium and allow the CO2 to bubble up and be collected. The CO2 will remain in this container until it diffuses into the water. It probably pays to have a current directed across the opening of the container to constantly bring fresh water in contact with the CO2.

If you have an external canister filter you can feed the CO2 directly into the intake tube. The time it takes the CO2 to exit the filter should ensure it is totally dissolved. 

Or alternatively you can mount a wide diameter tube (an old gravel vac is perfect) vertically in the aquarium and feed the outflow of your filter or power head down this tube. Feed the CO2 in from the bottom of the tube and allow it to bubble up against the flow of water thus keeping the bubbles of CO2 suspended while allowing fresh water to constantly mix with and dissolve the CO2. Some fiddling may be necessary to achieve the correct balance between tube diameter and flow rate. If the flow rate is too slow the CO2 will collect at the top of the tube, if the flow is too fast the bubbles will be forced out of the reactor. Additional gravel, foam, bio-media, etc. could also be placed in the tube to impede the upwards flow of the bubbles. A very small vent (such as a needle hole) at the top of the reactor is also advisable to allow any ``extra'' gases such as nitrogen (N2) and oxygen (O2) to escape. The source of these gases can be the original air in the generator which will be displaced by the production of CO2 or if there is a bubble of ``CO2'' in the top of the reactor dissolved gases may come out of solution in an attempt to equalise the concentrations of these gases between the bubble and the water.

There are probably countless variations on these themes. Where possible make reactors out of transparent materials so you can observe the rate at which the CO2 is being produced. 
 

 
Setting it going
The final step is to prepare the mixture which will produce the CO2. Dissolve 1 cup of sugar (any type of sugar will work) in about 500ml - 1 litre of water and pour into the generator. Dissolve 1/2 - 1tsp. of yeast (bakers yeast is available at any supermarket) in a small amount of warm (NOT HOT) water, add to the generator and top up to the required level with cold water. It is important that you do NOT overfill the generator. Once the yeast is activated and starts to produce CO2 a certain amount of ``froth'' will be produced. Enough head room must be left to ensure that this froth is not forced up the airline and into the aquarium. The addition of 1 tsp. of baking soda will help buffer the mixture and extend its working life. It also reduces the amount of froth produced.

If you use hot water to dissolve the sugar allow the temperature of the mix to reach room temp before connecting the reactor. As this water (and the air above it) cools and contracts it may start a siphon going, dumping aquarium water into the generator, if the production of CO2 has not yet started.

The above quantities are not critical so you can experiment with them if you wish. In general the quantity of yeast controls the rate of CO2 production, and the quantity of sugar controls how long the mix will last. Ideally you want a mixture which will produce CO2 at a steady rate for an extended period of time.

This mix should produce CO2 for at least 10-14 days (often longer). As it takes 12-14 hrs (depending on temperature) for a new mixture to start producing sufficient CO2 it is best to get a new batch started 24hrs before the old batch needs replacing. Alternatively you could have two generators producing CO2, one of which is replaced every week. This latter method probably provides for more stable conditions in your aquarium.
 

 
A few tips and warnings:
In order to maximize the benefit of injecting CO2 it is important that you reduce any surface turbulence as this will quickly allow dissolved CO2 to escape into the air. The best ways to achieve this are to, turn of any aerators, lower any spray bars below the water line, direct the current to run around the sides of the aquarium rather than from top to bottom and/or if you have air driven under gravel filters extend the uplift tubes above the surface of the water. Optimally growing plants will produce more O2 and saturate the water with O2 better than any other method you can try.

An increased CO2 concentration will not displace dissolved oxygen (O2) from the water. It seems to be generally accepted that a concentration of 15-10 parts per million (ppm) is ideal for optimal plant growth and that CO2 concentration must exceed 30ppm before becoming dangerous to fish (although the pH drop caused by suddenly injecting this amount probably will).

Do not unplug the CO2 system at night. At night, with the cessation of photosynthesis, plants become net producers of CO2 and consumers of O2 just like fish. This might lead you to believe that you should disconnect the CO2 at night to prevent CO2 building up. However the pH shifts caused by disconnecting the system will be much larger and more sudden than gradual change when the system is left connected. The rate of CO2 production is also dependent on temperature, so the generator will automatically produce less CO2 at night as it cools. (You may want to try sitting the generator near the warmth of your lights, but beware of potential electrical hazards should a spill occur.)

Sometimes a stringy white growth, of unknown composition, may start to grow on the airstone or end of the airline. It appears to be harmless to both plants and fish (certainly swordtails can eat it with no apparent side effects).

Finally, NEVER try and block or restrict the flow of CO2. The CO2 is constantly being produced so pressure will build up and eventually the generator will explode. At the best you will have one hell of a mess to clean up, at the worst physical injury could be the result.

 
Summary
Adequate light in conjunction with CO2 injection will enable plants to photosynthesize at or near optimal levels. The increased vigor of your plants as a consequence of this will be observed not only in their increased rate of growth but also in streams of oxygen bubbles being released from the leaves (this sight never ceasing to amaze us). In turn your fish will benefit from the increased oxygen concentration and the cleaner water produced by the plants. 

For those of you who are thinking this system is to good to be true, there is indeed at least one drawback. The increased growth rate will result in the need to spend extra time trimming many of your plants to prevent the tank becoming a jungle of growth through which you can no longer see your fish. In the case of some species of plants e.g. Hygrophila polysperma this trimming may need to be done weekly. Fortunately there is a ready market for these cuttings from pet shops and fellow club members.

The yeast system described requires some care and maintenance, but is an excellent and inexpensive way to experiment with CO2 injection. May your plants live long and prosper.