If you have a freshwater-planted aquarium, you have probably heard of CO₂ . It is the magic ingredient that all of the best tanks use. But why do you need to add CO₂ to your aquarium? And how do you add CO₂? This article will first explain the basic concepts, and then in the Advanced CO₂ Section delineate the individual components of advanced CO₂ systems and describe how the components work together.

THE BASICS
A planted aquarium is all about the plants, and what keeps plants growing is photosynthesis. Photosynthesis is the conversion of light into energy. For this to happen, other ingredients besides light need to be added to the equation. The full equation is CO₂ + Water + Light ==> Sugar + O2. In other words, plants need a combination of light, water, and CO₂ to get the sugar they use to grow.

On land, there is plenty of CO₂ in the air, and the focus is on watering plants and exposing them to light, but in an aquarium, there is plenty of water, so light and CO₂ are the most common limiting factors in photosynthesis. In nature, CO₂ is produced by fish breathing or respiring. Fish breathe out CO₂ through their gills just as humans breathe out through their lungs.

While there are plants in the water, there are usually not a lot in one place, and when there are, there is usually a surrounding area with very few plants. This is because the CO₂ from the fish in the water is only enough to support a relatively small plant population. In the aquarium, this means that if all you want is a lightly planted tank with low-demand plants, then CO₂ may not be necessary. But if you want a heavily planted tank that resembles an underwater forest, then CO₂ will be a necessary component of your aquarium.

There are a few ways of adding CO₂ to an aquarium. The simplest solution is to add one of the many liquid CO₂ additives. These additives are introduced in a variety of ways. Some claim to produce a compound similar to CO₂ that the plants use in place of CO₂. Others claim to generate CO₂ from the surrounding water. An example of these products is the Seachem Flourish Excel.

Beyond this there are some very simple CO₂ systems, such as Ista’s Disposable CO₂ Diffuser Set, that lack many of the components of a traditional CO₂ system. It is simply a compressed can of CO₂ that fills a plastic hood that sits in the aquarium. As the CO₂ contacts the water, it dissolves away, and the user presses a button on the top every few days to make sure that there is always plenty of CO₂ in the hood. The advantage of this approach is that it is simple, cheap, and hard to mess up, but it is only suitable for very small tanks (10 gallons or fewer) or tanks with only a few plants.

If you have an aquarium that has a large CO₂ demand, either because the tank itself is large or you have a lot of plants, then you will need a true pressurized CO₂ system. For more information about full-sized CO₂ systems please read the Advanced CO₂ Systems section that follows.

ADVANCED CO₂ SYSTEMS
CO₂ systems often intimidate and confuse aquarists. This is often because CO₂ products are often sold as individual components, and it isn’t clear how the components work together. To help make the process simpler and alleviate the confusion, we’ll describe the CO₂ components in the order that CO₂ passes through the system. Each component on the path will be explained individually. You can either buy each component separately or purchase an all-inclusive CO₂ system. We offer some such all-inclusive systems from Ista that will satisfy the needs of tanks up to 100 gallons.

Aquarists who have large, heavily planted tanks over 100 gallons, or who want the nicest system possible, will be better off purchasing the components separately.

The CO₂ Tank —The CO₂ tank is where the CO₂ is stored. For smaller systems, you can use the Ista Disposable CO₂ Cartridges. These are not refillable but need to be replaced once they are empty. These are suitable for small tanks. With tanks over 50 gallons, it is cheaper in the long run to use a refillable CO₂ tank.

CO₂ tanks can be grouped into two main categories: tanks using paintball-type fittings and tanks using standard CO₂ fittings. We carry three sizes of Ista CO₂ Cylinders that use standard CO₂ fittings (American CO₂ valve thread size CGA 320). We recommend for most applications using the largest one—the Ista Aluminum 3.0l Cylinder Full. This cylinder can be refilled at welding suppliers, some paintball shops, as well as liquor stores and home brewing suppliers.

You might even want to get a larger cylinder at a welding supply store, as filling your CO₂ tank is something you don’t want to do too often, as it requires some disassembly of your CO₂ system that may require recalibration once the system is reconnected. Also, some places that refill tanks will give a quantity discount. In general, refillable cylinders will save you a lot of money, and you’ll be better off in the long run purchasing the biggest canister you can afford and fit in your given space.

The CO₂ Regulator – The CO₂ regulator is what drops the pressure from inside the CO₂ tank down to a useable level and holds it there. If you were to open the valve on top of a CO₂ tank without a regulator, then a huge gust of gas would rush out, and the tank would soon empty itself. To stop this, we use a regulator to set the pressure. Regulators have a valve and either one or two gauges. The valve allows you to set the outgoing pressure.

If the regulator has a single gauge, such as with the Ista CO₂ Pressure Regulators, the gauge usually displays the pressure inside the CO₂ tank. This can give you an idea of when your tank is getting low on CO₂ and needs to be replaced.

If the regulator has a second valve like the Milwaukee MA957 CO₂ Regulator, it will display the pressure of the outgoing CO₂ as well as the pressure in the tank. You will typically want between twenty and forty psi coming out of the tank. Without this second gauge, you are just guessing the outgoing pressure.

The Needle Valve—A needle valve is what is used to control the amount of CO₂ that is entering the aquarium. Many people often confuse the needle valve with the pressure valve on the regulator. Needle valves are often attached to the regulator, like in the Ista CO₂ Controllers, but they can also be separate, like in the Ista Metal CO₂ /Air Flow Regulator - 3 Way. In some economy CO₂ systems, there is no needle valve, and all of the adjustments are made using the pressure valve. This is far less precise and more difficult to use than a needle valve. Once the pressure is set to the desired level, the needle valve lets us easily make precise adjustments to the actual flow rate of CO₂ .

The Bubble Counter—A bubble counter is a visual gauge of how much CO₂ is going into the aquarium. It’s a transparent container filled with water or mineral oil that is sealed with two connections. At the bottom of the container, CO₂ enters from the regulator, and at the top of the container CO₂ exits. As the gas passes through the fluid, it forms bubbles.

The simplest bubble counters look like the Ista Acrylic CO₂ Bubble Counter. A heavier-duty option is the Ista Metal Bubble Counter & Check Valve.

The amount of CO₂ used is described in bubbles per second. If two bubbles pass through the bubble counter in one second, then you are using two bubbles per second. Most people recommend starting at around one bubble per second, but using as many as six bubbles per second is not uncommon. Without a bubble counter, you are just guessing as to the actual amount of CO₂ you are using.

The Solenoid Valve—A solenoid valve turns the flow of CO₂ on and off electronically. Some regulators like the Milwaukee MA957 CO₂ Regulator have a solenoid built in. A solenoid can also be purchased separately and added to a system.

Why would you want to turn off the flow of CO₂ ? CO₂ is needed for photosynthesis, but plants do not photosynthesize at night. Because of this, injecting CO₂ at night is wasteful and also dangerous to fish. Too much CO₂ can literally suffocate your fish. This is why it is very important that the regulator and needle valve work well and keep the CO₂ injection rate where you set it. If you inject a large amount of CO₂ and continue to do so at night, there will be even more CO₂ in the water than during the day because the plants are not using it up at night. Most people plug their solenoid valve into the same timer as their lights. That way the CO₂ is only on when the lights are on. To get even more protection, the solenoid can be connected to a pH controller.

The Check Valve—A check valve in a CO₂ system ensures that water does not flow back down the line and ruin your regulator and solenoid valve. Some of the cheaper complete systems do not include a check valve. Every large-scale system should have a check valve, such as the Ista Spring Check Valve.

The CO₂ Tubing—CO₂ tubing is not the same as regular airline hose. CO₂ hose is much sturdier and less prone to leaking and kinking than air hose and it is better at handling the pressure that forms in the lines from having compressed gas travel through it. The Marina Silicone Airline Tubing is recommended.

The Diffuser or Reactor—Diffusers and CO₂ reactors help dissolve CO₂ gas in aquarium water. If the CO₂ just bubbles up to the surface and dissipates into the air, the plants never get a chance to access it. You want CO₂ to dissolve into the water, where it is available for the plants.

Diffusers like the Ista UFO CO₂ Diffuser have a ceramic disk with fine pores that dissipate the flow of CO₂ into very fine bubbles. The small bubbles have a greater surface area than the larger bubbles that would form without the diffuser. The gas only dissolves when it makes contact with the water. Because there is more CO₂ contacting the water there is more CO₂ dissolution.

This small disk is placed in the aquarium as far down as possible. Placing the diffuser at the bottom of the aquarium increases dissolution by increasing the amount of time that it takes for the gas to reach the surface. The longer the gas is in contact with the water the more CO₂ dissolves.

Reactors also help to dissolve CO₂ , but they do it in a different way. Most reactors such as the Ista Max Mix CO₂ Reactors are pressurized containers. These containers are usually placed in line with either a canister filter or the return from a sump filter. The reactor adds the gas to fast-moving swirling water and helps the CO₂ dissolve by limiting its exposure to air and prolonging the contact time between the CO₂ and aquarium water. Most people agree that CO₂ reactors are more efficient at dissolving CO₂ than diffusers, and they offer the added benefit of not having any equipment in the aquarium.

pH Controllers – pH controllers are used to control solenoid valves and prevent the accidental suffocation of fish. A pH controller such as the Milwaukee Instruments pH Controller, MC122 measures the pH of the aquarium and turns an outlet on and off based on what the pH value is. When using a pH controller to control CO₂, you want to have the pH controller turn off the CO₂ whenever the pH drops below a value you set and then turn back on once the pH rises back up. This is important because CO₂ is acidic and adding CO₂ lowers the pH of the aquarium water.

Set the controller to a value of around half a point lower on the pH scale than the aquarium naturally sits. For example, if an aquarium naturally sits at a pH of around 7 then we would want to set the controller to 6.5 so that the controller turns the CO₂ off when the pH is below 6.5 and back on once it rises above 6.5. When this is done correctly the controller will turn the CO₂ system off at night because the plants stop absorbing CO₂ and the pH drops as CO₂ accumulates, and then turn the CO₂ back on once the plants start to absorb the CO₂ and the pH rises. The advantage of the pH controller over a timer is that if you accidentally open up the needle valve too much, and the aquarium starts to flood with CO₂, the controller will turn off the CO₂ before the fish suffocate. Without this extra layer of redundancy, the fish would die if you made this mistake.

As we can see there are a lot of parts to a CO₂ system. The quality of these parts, as well as the presence or absence of each piece is what causes there to be such a variation in the price of CO₂ systems. If you are looking for a system, Ista offers Complete CO₂ Systems at different price points.

If you wish to put together a complete CO₂ system on your own, there are a number of choices that have been described in this article. Here are two recommended paths you can go by that allow for some choices based on your budget and size of aquarium.

Path #1—The Simple Path
CO₂ Tank >> Milwaukee MA957 CO₂ Regulator (regulator, bubble counter, needle valve, solenoid valve) >> CO₂ Tubing >> Check Valve >> CO₂ Tubing >> Reactor. Run the water line from the canister filter or pump in the sump through the reactor. Plug the Milwaukee MA957 into a timer and run when the lights are on.

Path #2—The Best Path
CO₂ Tank >> Milwaukee MA957 CO₂ Regulator (regulator, bubble counter, needle valve, solenoid valve) >> CO₂ Tubing >> Check Valve >> CO₂ Tubing >> Reactor. Run the water line from the canister filter or pump in the sump through the Reactor. Plug the Milwaukee MA957 into a pH Controller and have the controller turn off the CO₂ when the pH drops to a set point.