Nearly all the problems that I'm emailed about regarding algae and poor plant growth can be attributed to a problem with their CO2 setup. Carbon makes up around 40% of a plant's dry weight so having a plentiful supply is vital for the health of your plants. Due to advancements in lighting technology planted tanks these days run much higher lighting levels compared to those of only a few years ago. If you have low lighting levels then you may not need CO2 addition, but if you run two or more florescent tubes above your tank the the chances are you probably will.

Also tied in very closely with CO2 are water flow in the tank and oxygen levels. I shall discuss these more later under application.

Sources Of Carbon

There are four main sources of Carbon available for the planted tank. They are CO2 gas from a pressurised cylinder, CO2 from a fermentation bottle, a bottled liquid containing a carbon source and CO2 from electrolysis.

Bottled Carbon Dioxide Gas
This uses a pressurised bottle of compressed CO2 to which a regulator is attached. A needle valve is used to control flow via an inline bubble counter. It is the preferred method as it's very easy to maintain CO2 levels in the tank. It can also be turned off at night using a solenoid when it's not needed to preserve gas. The main drawbacks are finding a safe place to store the bottle of CO2 and the initial cost of purchasing all the equipment.

Fermentation Bottle
A mixture of water, sugar and yeast are placed in a sealed bottle except for a tube running to the tank. As the solution starts to ferment CO2 is formed. Some people have had great success using this method but it is very hard to control CO2 production because of temperature and also the way the fermentations starts off slowly, then speeds up only to slow down again at the end. Having more than one bottle going with staggered starting times can help smooth out CO2 delivery. Even though costs are very favourable, it's unreliability means it's not really a recommended solution.

Bottled Liquid Carbon
Gaining in popularity are bottled liquids containing a source of carbon. The main two are Seachem's Flourish Excel and Easy-Life's EasyCarbo. These are very well suited to the smaller planted tank due to the purchase cost. As they aren't as efficient a carbon source as gaseous CO2 is, there may be problems if you have strong lighting levels. One positive side effect is that they are mild anti-algae agents so will help keep algae at bay.

Electrolysis
This uses a block of carbon that is placed in the tank and has an electric current applied to it. This creates the CO2 gas by electrolysis. In theory it's a great idea but in reality it's very hard to achieve decent CO2 levels and it can be very messy. My advice is to avoid them.

Measuring CO2 Levels

This is one of the hardest parts with CO2 systems. The method that you measure tank pH and KH and compare your results in a table to obtain the CO2 level used to be very popular, and still is sometimes, but can be very inaccurate depending on tank conditions. This is due to KH test kits not actually measuring carbonate hardness at all, but alkalinity instead. This means that any buffering compound in the tank will affect the results.
The best currently used method is to use a drop checker with a 4dKH solution and adding a few drops of pH indicator that uses bromothymol blue. Response time is slow but this normally doesn't matter much if you supply a constant rate of CO2. A light green colour is indicative of around 30ppm CO2 which is the recommended level to have. Blue means you have too little and yellow means you have too much possibly asphyxiating your fish.
If you use one of the bottled sources of liquid carbon then you are unable to measure levels. Dose as recommended on the label.
At the bottom of this page I have detailed how to make your own 4dKH solution

CO2 Application

You've got your fermentation bottle or CO2 cylinder, regulator, needle valve, bubble counter and tubing all in place and now you need somehow to dissolve the CO2 in the water. There have been many great ideas over the years but I'll just outline the main ones here.

Sintered Glass Diffuser
These come in varying shapes and sizes with the main principal of forcing CO2 through a sintered glass/ceramic disc to create a fine mist of bubbles. As the bubbles have a very high surface area to volume they dissolve very quickly. Very good for using in small tanks but in large tanks you will probably find that more than one is needed to obtain good CO2 distribution. Best used with pressurised CO2 systems due to the pressure required to force the CO2 through the disc.

External Reactor
These run inline with the return tube from the filter outlet to the tank. They are basically a plastic tube where water enters from the top with CO2 being bubbled in from the bottom. As the bubbles of CO2 are going against the flow of water they remain in contact with the water for longer. Sometimes a filter mesh or bio-balls are placed in the tube to slow the bubble's rise to the top. They require a fairly large filter to provide enough flow to dissolve the bubbles quickly, so mainly for this reason they are best suited to large tanks.

CO2 Ladder
These are placed inside the tank and come in many different shapes ranging from spirals to ladders. The idea of these is a bubble of CO2 enters at the bottom of the unit and is made to travel a long way before it reaches the surface. In a spiral the CO2 bubble goes round and round, but in a ladder it goes back and forth. The idea is to keep the CO2 bubble in the water as long as possible. They can be quite wasteful of CO2 and also don't work very well in larger tanks.

Others
Some people inject CO2 directly into their filters, but this is a bad idea as not only do you run the risk of an air lock but also CO2 attacks the filter's rubber seals.
Attaching the CO2 tube to the inlet of a powerhead. Can work well but also can be a bit noisy.

One other vital piece of kit is a non-return-valve. This is to prevent water getting back to the regulator/needle valve/solenoid and damaging them or on a yeast bottle the tank water draining into the bottle.

Start adding the CO2 slowly and keep a good watch on your drop checker to make sure it doesn't go yellow. A bubble rate of one bubble every two seconds is a good place to start. Drop checkers can take an hour or two change colour so patience is required. Keep increasing the bubble rate slowly until a light green colour is achieved. It is possible that several bubbles a second are required in a large tank. It may take several days for levels to stabilise. Once stable is is important to keep CO2 levels stable day in day out. This is because plants adjust to CO2 levels in their surroundings. If the levels are constantly changing then the plants also have to keep adjusting which uses up energy.

Some people like to use a solenoid to turn off CO2 at night. This is fine but you must turn the CO2 on about two hours before the lights come on to make sure that CO2 levels are at optimum. You can then turn the CO2 off an hour or two before lights off as the plants demands aren't as great and normally there is plenty of CO2 present to last.

I have left the two most important things to last. They are flow and oxygenation. These are vital and often overlooked aspects of keeping a CO2 enriched planted tank.
Flow
As a guide try to aim for about 10x or more the filter's rated flow for your tank volume. What you are looking for is that all the plants are gently swaying. Without this the CO2 distribution will be poor resulting in poor plant growth and possibly algae. To supplement flow you can add an extra water pump if you can't achieve the flow from your filter. I have found the best method for achieving good all-round flow is by aiming the water return up slightly along the water's surface towards the front glass. This creates surface movement, which I'll come to in a minute, but also once the water hits the front glass it is forced down to the substrate and then travels along the substrate towards the back of the tank.
Oxgenation
As mentioned above the water return needs to create a bit of surface movement to help oxygenate the water. It used to be said that it was best to not disturb the water's surface in a planted tank as CO2 will be lost. But I and many others find that the plants and fish do much better with good water surface movement. The downside of this is that a bit more CO2 is required, but the benefits greatly outweigh the small extra cost. An extra bonus is that algae don't like lots of flow and higher oxygen levels.

It is easier to keep CO2 enriched small tanks compared to large tanks. This is mainly down to water flow and CO2 distribution. Keeping this in mind if you have, or are planning to have, a large planted tank then getting your water flow working well is of paramount importance.

DIY 4dKH Solution

Accurately weigh 6.0g of Sodium Bicarbonate and add to 5 litres of deionised water to make a 40dKH solution.
Mix 10ml of the 40dKH solution with 90ml of deionised water to give you 1 litre of 4dKH reference solution.