Carbon is the molecular backbone of all living things, and plants are roughly 90% Carbon and Oxygen. This fact alone should impress upon you the importance of providing these molecules, which you do in the form of carbon dioxide or CO₂.

Many beginning aquatic plant growers believe that insufficient light is the cause of poor plant growth. Actually plants receiving low light and good levels of CO₂ will do better than plants receiving good light and low levels of CO₂. If you don’t learn anything else from this article, learn that it is more important to invest time, effort, and money into making sure your plants are receiving a good supply of CO₂ before anything else.

Oxygen "Pearls" are formed when plants receive optimal growing conditions which includes adequate CO2.

Another error beginners make is believing that the fish will give off enough CO₂ for their plants. It doesn’t matter how many fish you have, they probably don’t give off enough. The exception is in some very slow growth, low light tanks. In those tanks the CO₂ from fish and that which is being absorbed into the water from air stone agitation may be minimally adequate. This exception is exactly that, an exception. For most people aspiring to grow aquatic plants, you must add CO₂.

A final mistake is using an air stone. Surface agitation causes CO₂ to be driven out of the water when it needs to be conserved. This might seem contradictory to the statement in the previous paragraph, but there is a difference. In a tank with no supplemental CO₂, the tank becomes so severely depleted that the small amount percolated into the water with the air stone is at least providing a nominal addition. In a tank with supplemented CO₂, the air stone drives out what you have added, causing a loss. This starves the plants in addition to wasting money from repeated CO₂ tank refills.

Thoughts on the amount of CO₂ needed vary from just a little to around 30ppm. Early on people were mostly using 15ppm, but in the last few years 30ppm has become more common. You can determine how much you have by testing for CO₂ directly or by using the KH/pH table. A CO₂ test kit can be expensive and using the table is fairly accurate. Therefore, most people don’t spend money on a CO₂ test kit.

Carbon dioxide added to water causes it to become more acidic. As previously discussed, bicarbonate added to water creates a buffer and causes it to become more basic. Therefore, since KH is roughly a measurement of bicarbonate, if you know what your KH and pH are, you can mathmatially estimate the CO₂ level using the KH/pH table. This method is not without error. There are chemicals other than bicarbonate and CO₂ that can alter your water's pH, but it’s a good estimate nonetheless.

Using the table below, you can see that if you have a KH of 4 and a corresponding pH of 6.8, you have an estimated CO₂ level of 18.7 ppm. In this case, to get the CO₂ level to 30ppm the hobbyist would increase the CO₂ flow to decrease the pH down to about 6.6. Occasionally a beginner is confused and wants to add some chemical to decrease the pH. This doesn't make sense. The purpose is to get the optimal amount of CO₂ which is not increased by adding chemicals but only by adding more CO₂.

I will mention several times in this article how it is important to maintain appropriate levels of CO₂ regardless of of how much light you have. I want to clarify that statement now. If after additional research you decide you need 30ppm of CO₂, then you should work to maintain 30ppm CO₂. No matter how much light you have or fertilizer you add, you need 30ppm CO₂. Yes, in high light, CO₂ is used up faster, but you don't want to add more as this will decrease you pH. In low light, the plants use less CO₂, but if you add less, the plants won't have enough, and your pH will increase. As long as you maintain 30ppm, there will be plenty to go around, and your pH will remain stable. Therefore, maintain appropriate levesl of CO₂ regardless of all other factors.

There are many ways to get CO₂ into the water. Thankfully, these days it is much cheaper than in the past. Four methods will be briefly covered here. They are yeast fermentation, which is also referred to as DIY (Do-It-Yourself), organic carbon, carbon electrolysis, and compressed CO₂.

DIY CO₂ involves adding yeast, sugar and water to a bottle which is sealed and has a tube leading from the bottle to your tank. The yeast uses the sugar as food and gives off CO₂ as a waste by-product. The output end of the tubing is usually placed into the intake of a filter which helps dissolve the exiting CO₂ into the water as it passes through. As you can imagine, yeast in a bottle doesn’t give off a great amount of CO₂, so this method is usually used for small tanks or medium size tanks with lower light and slower growing plants. It’s advantages are that its very cheap, easy, and works excellently well. A brew of DIY yeast culture will last about two weeks. A caveat to this method is suspending the sugar in gelatin. The yeast does not have access to all the sugar at once. Therefore, it is digested at a constant rate, which provides a slower and more consistent CO₂ output. The advantage of doing it this way is that it will last 2 to 3 months. The disadvantage is that the output of CO₂ is much less, so you are limited to very small aquariums, or you must add additional bottles of yeast. Other disadvantages to the DIY method in general are that it is fairly labor intensive and can be quite messy.

Many plants are able to extract carbon from some small organic compounds. This ability is taken advantage of with the Seachem product Flourish Excel. This is usually a good choice for smaller tanks because of cost prohibitions and because it is just not feasible in large tanks where so much CO₂ is needed. Five milliliters of Flourish Excel is added per 10 gallons of water 2-3 times a week. People have complained of this product killing fish, but it only happens when you use more than the recommended amount. Also, it is safer if you dilute it in a little water before adding it or if you pour it in near your filter output. I believe that the only people that have killed fish with this product are people that used it incorrectly. I have used in excess of the recommended amount and poured it directly on top of gathering fish who thought they were about to be fed with no deaths. (As a side note, Flourish Excel is the best thing there is for killing black brush algae in my opinion.) Tanks with organic matter in the substrate can also benefit from this method. As the organic molecules decompose and float up into the water, some plants are able to extract CO₂ from them. For more in-depth information on this topic read Ecology of the Planted Aquarium by Diana Walstad, which is available at the AGA Bookstore.

Carbon electrolysis involves a carbon block that is connected to electrodes. Electricity generates a reaction between the aquarium water and the carbon that causes CO₂ to be released. This is another method that is best for smaller tanks or medium-sized tanks with lower light and slower growing plants. The device gives off small amounts of CO₂, so in large tanks or brightly lit medium size tanks, the cost of replacing the blocks may more than justify the purchase of a compressed gas system. Advantages of this system are that it is very easy to use and works great. It's disadvantage is that it can be expensive. Carbo-Plus is the brand name of one of several devices that act similarly.

A compressed (or pressurized) CO₂ gas system is best for most situations. It is less time consuming, less messy, and in the long run usually cheaper. The only situation for which it might not be the best choice in is small- or medium-size tanks with low light and slow-growing plants. In those instances, it is a good choice, but it is probably more than needed. A compressed CO₂ system consists usually of a bottle of CO₂ gas, a regulator, a needle valve, and something to help dissolve the CO₂ into the water. The system can also contain a solenoid valve and pH controller which will be discussed later. CO₂ is allowed into the system via the regulator. The needle valve reduces the large volume of CO₂ from the regulator into a smaller controlled release of gas. Sometimes at this point a bubble counter is added to help gauge the rate CO₂ is flowing. Finally it all runs into a diffuser or reactor. A diffuser turns the stream of CO₂ into tiny bubbles, which are released directly into the tank. The bubbles dissolve into the water as they rise to the surface. A reactor is plumbed into the tubing from your filter or connected to a small pump. CO₂ is bubbled into the reactor and dissolves as water passes through. Reactors are much more efficient, but they are also more expensive. Some people run the outflow of CO₂ bubbles into their filter intake, which dissolves them as they pass through. Sometimes, however, CO₂ can build up and stall the filter and/or cause “burps” of CO₂ to be released.

With a compressed gas system, you can easily add too much CO₂, bottom out your pH, and suffocate your fish. Therefore, you must control its flow into your aquarium. There are two processes for maintaining safe CO₂ levels: the manual and the automatic system.

With the manual system, CO₂ is slowly and constantly diffused into the water. This is the system employed by most aquarists. It is reliable, and the equipment needed is cheaper. Here are some factors to consider with this method. First, during the day so much CO₂ is absorbed by the plants that the water can become deficient if not absent of CO₂ after a few hours with the lights on. Second, the opposite situation is a problem at night. Since the CO₂ is always going, it can build up, and levels can become too high. To remedy this, many people use an electric solenoid valve and timer to turn off the CO₂ flow at night. Alternately some use an air stone at night with a timer to drive excess CO₂ out of the water. Still again, since the water usually becomes so severely depleted of CO₂ during the day, many people leave the CO₂ on 24 hours because if the flow is small enough, it doesn’t build up to critical levels anyway. I repeat that the manual system is the method used by most aquarists. While it seems to be fraught with hazards, it obviously works well. In recent times single unit regulator, solenoid, and needle valve combinations have been available at a reasonable price. This has made option number one above almost a no-brainer. I personally don't recommend leaving the CO₂ on 24 hours a day.

The automatic system adds a pH meter/controller and an electric solenoid valve to the manual method. Remember when CO₂ is added to water, it makes it more acidic. The pH meter is set to a certain level, and when the pH goes slightly above that level, the controller (meter) opens the solenoid valve, and CO₂ is added to the aquarium. When the pH goes back below the set level, the controller (meter) will shut off the solenoid valve and CO₂ injection is stopped. Many like this method, but the additional cost of the meter/controller and solenoid is prohibitive. Since the manual system is reliable, it is often argued that the automatic system is a waste of money. However, there are those that enjoy the accuracy, reliability, and gadget fun provided with automation. Also the price of pH controllers has come down in recent years.

Basically you will test your KH. Then with the manual system, you will test your pH very often over the first few days and correspondingly adjust the needle valve to increase or decrease the flow of CO₂ until you get the pH in the range you need. With the automatic system, you set the pH colntoller to the pH you want, and it monitors the water and adds CO₂ only when you need it.

You should be concerned with the safety of compressed gas containers in the home. Should the bottle be tipped over and the regulator broken, a CO₂ cylinder can become a dangerous missile. This is particularly an issue when children or pets are involved. Use extra caution with bottles of compressed CO₂ gas.

There is a last method by which plants can obtain carbon that will not be discussed in this article called biogenic decalcification. In this process plants break down bicarbonate to utilize the carbon and oxygen within that molecule. Plants that originate from hard waters take the best advantage of this process. This makes sense because it is bicarbonate that makes the water hard. The application of biogenic decalcification to the home aquarium is limited. An excellent discussion of this topic can be found in Ecology of the Planted Aquarium by Diana Walstad, which is available at the AGA Bookstore.

So what are these pretty little bubbles that are coming from your leaves now? No, it's not excess CO₂. It is oxygen. In a healthy planted aquarium with supplemented CO₂, the plants put off so much oxygen that the water becomes unable to absorbe any more. The oxygen collects underneath the leaves until it coalesces into bubbles which float to the top of the water. The effect is rewarding and beautiful. It makes all the trouble of setting up CO₂ worth the effort.

DIRECT LINKS INTRODUCTION | WATER | FILTRATION | CO2 | LIGHT | SUBSTRATE NUTRIENTS | COMPUTER | MISCELLANEOUS | CONCLUSION