A Field Guide to Aquarium Carbon Dioxide Systems

People who visit my living room are often struck by the sheer, jungle-like lushness of the vegetation in my 125-gallon aquarium. The tank has such a profusion of plant life that its fish sometimes fight for the clear spaces or disappear for weeks on end in the thickets, living as they would in only the most abundant natural settings. This is a far cry from the aquaria I maintained as a child, when the only plants I could keep alive were the most beginner-friendly, least demanding species, if even then. Perseverance got me to my current skill, and a key part of that perseverance is learning my way around more advanced tools of the aquarist trade. And for someone who takes great joy in aquatic plants, that means carbon dioxide (CO2).

All in the Balances

Plant metabolism hinges on carbon dioxide. Via photosynthesis, plants and their algal cousins convert carbon dioxide into sugar using energy they collect from light, underpinning most of our planet’s ecology in the process. The good news for aquatic plants is that water has a phenomenal capacity for carbon dioxide, far in excess of the levels typically found in air. (This is in contrast to water’s capacity for oxygen, which can be upwards of 20 times lower than air’s and explains some quirky animal biology along the way). The bad news is that, because carbon dioxide enters water through simple diffusion at the water’s surface and through the exhalations of aquatic life, the actual level of carbon dioxide in an aquatic setting is rarely much different from that found in nearby air and can be much lower in still water with few animals in it. When simple diffusion is the entry method, carbon dioxide exits the water at the same rate that it enters, which means water’s high capacity for carbon dioxide becomes mostly irrelevant. Fortunately, these two facts dovetail rather neatly once humans get involved. By administering external carbon dioxide, an aquarist can make use of that excess capacity and provide more of what their plants crave.

It is not quite that simple, however. A planted aquarium effectively has three resources that need to be kept in balance: light, carbon dioxide, and nitrogen, which here mostly means the nitrogenous waste of fish and other animal life.

A mistake many beginning aquarists make is overfeeding their tanks. This creates an overabundance of nitrogen in their systems. With beginner lights that are not especially bright and no CO2 injection, any plants they might include cannot take in that excess nitrogen, but algae can. These aquarists then contend with the bane of most fish tanks: an algal bloom. Over-abundant algae are an aesthetic issue, encrusting every surface that gets enough light for their minimal needs, but they can also become a practical problem: when they respire at night, they drain oxygen from the water, and enough of them can make the tank uninhabitable by anything else.

Similarly, a key part of the photosynthetic process is light. Without light in comparable abundance to any carbon dioxide on offer, plants cannot take advantage of more CO2. The excess, then, ends up feeding algae and/or reducing the pH of the system, which might be a problem for fish that prefer more alkaline water and especially for snails, crustaceans, and other shell-building animals that might be present. Conversely, too much light in a system without CO2 and nitrogen to match can cause, you guessed it, an algal bloom.

This means that adding carbon dioxide to a planted aquarium is best done in tandem with getting high-end, plant-optimized lighting for one’s system, to make sure that the resources one is putting such effort into providing are actually available to one’s plants. In turn, the balance between the three key resources must be maintained, by keeping an eye on feeding and stocking levels, keeping the carbon dioxide flowing at a good rate, and making sure the lights match.

Lesson one: If you’re getting into CO2 injection in your planted tank, also get higher-end lights than you’d use for a non-CO2 system.

A Nymphaea water lily flower, purple and resembling a daisy, emerging from the water in an aquarium.
Carbon dioxide enabled this water lily to bloom in my aquarium.

General Layout

Getting set up with carbon dioxide for aquaria requires four things: a CO2 source, a way to regulate dosing, tubing that connects that source to the aquarium, and a distribution system within the aquarium. All CO2 systems, no matter how complicated or what their price point is, have these four parts. Decisions made at early points in setup can influence what forms these parts take, so, let’s start from the source.

Three Ecosystems

There are three different carbon dioxide sources one can use in a planted aquarium, each of which effectively piggybacks off another hobby that has uses for carbon dioxide. These sources occupy three different price points, levels of complexity, and general success at achieving what planted aquarists are trying to achieve, and picking one dictates what options a person has thereafter for the other parts of their CO2 system.

  • Yeast fermentation produces carbon dioxide via yeast consuming sugar and exhaling it. Systems that rely on yeast fermentation are the most basic possible CO2-injection setups and can be built at home from leftover airline tubing and soda bottles. They are also commercially available, and the ones available for purchase typically come with Venturi pumps, better tubing, and other niceties that make them more effective. In one of these systems, a small quantity of yeast is added to a large quantity of sugar and warm water and the yeast’s metabolism releases CO2, which is then piped into the aquarium. These systems typically must be refilled monthly as the yeast poison themselves with their other waste products and die, and since that waste product is ethanol, these systems should also be kept out of reach of children. (This is the first few steps of making bargain-basement rum, essentially, prior to distilling it.) Yeast fermentation systems are difficult to control, since they depend on a biological process, and don’t produce much CO2 even when they are working at their best, so they are the safest carbon dioxide system to add to an established setup when fancy lights are not available.
  • Paintball CO2 uses the pressurized gas canisters sold for use with paintball, airsoft, and similar compressed-gas projectile games rather than a jug of water, sugar, and yeast. A whole system of gas flow regulators has been invented to make these canisters usable for aquarists, since they are already widely available for their intended hobby. CO2 vessels meant for use with paintball tend to be relatively small, reasonably standardized, and built to be disposable, so the regulators that use them can be smaller and less expensive than those used below, but this also means that the expense of this method adds up over time as those canisters need to be replaced.
  • Brewing CO2 uses industrial-style compressed gas cylinders meant to deliver carbon dioxide to people brewing beer. This style of cylinder is also used to deliver oxygen for people on breathing assistance, deliver oxyacetylene and hydrogen for welders, and various other applications, so their cylinder sizes, valve sizes, screw threads, etc are standardized and easily looked up. The cylinders themselves range from “fits in an aquarium cabinet” to “the size of a person,” allowing this method to match even the most extreme planted-aquarium situations. In the long term, using brewing-CO2 equipment provides the greatest efficiency, since the cylinders can be taken to home-brewery supply stores to refill and don’t have disposable parts, but it also has the largest upfront cost.

Lesson Two: The CO2 source dictates almost everything else about the system.

Stay Regulated

Regardless of the source chosen, the chance that carbon dioxide will exit that source and reach the tank at the perfect rate continuously forever is essentially nil. Getting as close to that as possible requires some manner of gas flow regulation.

  • For yeast fermentation, very little control is possible. Systems that feature Venturi pumps usually enable the rate of water flow within the pump to be adjusted and this affects the rate of CO2 diffusion, but the whole system is ultimately dictated by the rate at which the yeast generates CO2, and that varies with the yeast’s life cycle.
  • Paintball CO2 and brewing CO2 systems use formal gas regulators with valves and gauges. Because of the different sizes of the connection points to their respective carbon dioxide sources, the regulators used in these two systems are different and it is rare that one meant for one source can be used with the other.

The best gas regulators for aquarium use have two gauges (one reads the outflow pressure, the other the pressure in the gas canister), needle valves and bubble counters for making fine adjustments to CO2 flow rate, and solenoid valves that can shut off CO2 flow at night (if connected to a timer) or during a power outage. These systems can deliver enough CO2 to affect water pH if plants aren’t using it, such as when plants are unlit for extended periods, so being able to shut off gas flow on these occasions is useful. Two-gauge regulators are common enough in industrial applications, but the addition of needle valves, solenoid valves, and bubble counters is peculiar to aquarists. These parts can all be purchased separately and attached with a good wrench, some know-how, and some elbow grease, but all-in-one aquarist-oriented regulators are also on the market and save some headaches, fine-tuning, and leak-checking. This regulator is typically the most expensive part of getting into aquarium carbon dioxide, alongside the lights.

Lesson Three: Invest in a good regulator.

Tubular

Once gas exits the source and regulator, it needs to travel to its destination, and that means tubing. Yeast fermentation systems often use ordinary airline tubing of the sort used for air stones and other accessories that create bubbles of air in an aquarium, but this tubing is gas-permeable and does not lead to optimal CO2 delivery. Savvy keepers, especially those using paintball or brewing CO2, use dedicated gas tubing that is usually black in color and much stiffer than airline tubing. It is important not to induce sharp bends or kinks in this gas tubing, as this compromises its integrity and can cause leaks. This gas tubing typically has the same diameter as airline tubing.

Lesson Four: Proper gas tubing saves trips to get more carbon dioxide.

End of the Line

At last, we get to the part where the carbon dioxide reaches the dihydrogen monoxide. Here, there are many options to suit many applications.

  • For yeast fermentation systems, CO2 flow is rarely high enough that it especially matters what system is in use. Commercially available kits typically include an electric Venturi pump that mixes the gas outflow with water at an adjustable rate, but completely passive systems are at least theoretically possible.

Paintball CO2 and brewing CO2 systems can use either glass diffusers or in-line diffusers.

  • Glass diffusers have several shapes available, including small inverted bells that fill with gas and shower-head-like shapes that disperse their gas into small bubbles. These connect directly to the gas tubing and can create interesting visual effects via the mist they often generate. In a properly clean tank, the diffusers themselves are often nearly invisible and don’t usually complicate the look of a tank. These are ideal for small tanks due to their small and unobtrusive nature but can struggle to deliver enough CO2 in larger systems.
  • In-line diffusers are Y-shaped structures that are placed inside the outflow lines of external canister filters. The stem of the Y faces the filter itself, one arm of the Y stays in line with the filter tubing, and the other arm of the Y connects to the gas tubing and, from there, to the gas source. These are ideal for larger tanks, which are disproportionately likely to use external canister filters, and have the additional advantage of keeping the entire CO2 system out of sight wherever the filter is.

In-line diffusers are widely considered the gold standard for carbon dioxide in planted aquaria, especially when connected to brewing-style CO2 cylinders.

Accessories

When working with carbon dioxide in aquarium settings, the system itself is only most of what’s needed. One other accessory that is helpful is a CO2 indicator, which takes the form of a small glass vessel containing pH indicator fluid affixed to the inside of the tank. The indicator’s shape keeps water from getting inside, which keeps the (non-toxic) indicator from escaping, and the air inside will be in chemical equilibrium with the aquarium water. The color of the indicator provides an at-a-glance read of the pH of the system, which is critical information. Carbon dioxide forms a weak acid in water, so depending on the buffering capacity of your aquarium, it is possible that overzealous CO2 dosing can have negative effects on water chemistry and animal health. Similarly, a slow increase in tank pH might be an indication that the CO2 source has run out and needs refilling or that there is a leak. Indicators are best placed at the far end of the tank relative to the CO2 distributor.

Another accessory that can be useful is a gas line splitter. Similar to splitters used with airline tubing, these splitters can enable a single source and gas regulator to serve multiple nearby aquaria. Since the gas regulator is generally the most expensive part of a CO2 system, this can save a tidy sum. (This is, alas, no help with the other most expensive part of being a planted aquarist, the lights.)

Happy Planting

With luck, this rundown of what’s involved in getting into aquarium CO2 is helpful to someone. Let me know how it goes for you.

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A Field Guide to Aquarium Carbon Dioxide Systems
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