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 five 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 to 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 much 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 air line 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.