Sources of Carbon for Aquariums
There are two common sources of CO2 and one source of organic CO2. CO2 is most efficiently purchased in pressurized bottles from specialty gas stores (like Praxair, Air Liquide) as well as beer supply stores and welding supply stores. The standard economic grade works just as well as any other higher grades, there is no benefit to spending more on a higher grade of gas. Most often, the company will lease you a bottle and when it is empty, you bring it back and exchange it for a full bottle. However, some companies will ask that you buy a tank instead of lease. Research this carefully, as we have found that other companies may not be able to refill your privately owned bottle on site, or are unwilling to fill it due to liability.
Hobbyists with smaller tanks may be able to make due with Do-It-Yourself (DIY) CO2, also known as the How To Make CO2 Yourself method. There are many versions of this system, and they share a common trait. They use baker’s or brewer’s yeast to ferment sugar to produce CO2. Now, several manufacturers have taken this idea and turned it into a suave, eye-catching design. Economic hobbyists merely mix up yeast, sugar, and water in a 2-L pop bottle and attach airline tubing to harness the CO2. Whichever ‘system’ you use, I feel it necessary to point our a few issues. The first, always use a check valve to prevent yeasty water from flowing through the air tube and into your tank along with the CO2. Yeast will foul your aquarium water. Second, this method is good for tanks up to about 20 gallons in my opinion; after this point, you use too much sugar for it to be economical.
There are other methods for CO2 generation; Carbo-plus is one such system. It is relatively unheard of in my area, however, it is worth of a few sentences. Hobbyists with high KH report that this system works better for them than without high KH, suggesting that the system is generating CO2 from bicarbonate (HCO3-). In much of North America, our water is very soft, so this system would not work unless you add bicarbonate, and replenish it as it is used up.
Finally, the liquid systems! When these first appeared on the market, my initial thought was, yeah right, how could this possibly work? Specifically, Flourish Excel was the first brand I encountered. However, after testing it out, I was forced to admit it does work to provide plants with carbon. The product specifications indicate it is a polygluteracetal compound, which translates as gluteraldehyde in water. However, Seachem, who manufacturers Flourish Excel, has stated that the product they use is similar in structure to gluteraldehyde (it is called an isomer). Hobbyists began rushing out to add gluteraldehyde to their aquariums (myself included). However, gluteraldehyde is a nasty chemical, many people are allergic to it, and it is used in hospitals as a sterilant (one reason Excel is good at killing certain types of Algae). It also has a short half-life, meaning it will react quickly and is not able to be stored for a long period of time. Flourish Excel has been ‘stabilized’ and seems to retain its potency for much longer than my experience with gluteraldehyde.
In my opinion, Flourish Excel is useful for small tanks with low to medium light. However, I have found it is not sufficient for large tanks, or those with high light. The only system I have ever had success with for those tanks is bottled CO2.