The Complete CO2 System

This post is for all of the planted aquarium newbies who are just getting started with CO2 and the planted aquarium, or for those more experienced hobbyists looking for a review. Setting up a complete CO2 system for the first time can be a little intimidating, we hope that this post will help answer some questions and alleviate a little of the mystery involved before you dive in and set up your very own CO2 system.

Our last post was all about the CO2 regulator, the backbone of the CO2 system; so make sure to read our previous article on the regulator.

The CO2 Pressure Regulator – The purpose of the CO2 regulator is to reduce the high pressure inside of a CO2 cylinder to a lower, usable pressure that can be dispensed into the planted aquarium. The pressure reducing regulator takes a pressure of 800 – 1000 PSI (pounds per square inch) from the CO2 cylinder, and regulates it to provide a controlled, reduced pressure output in the range of 1 – 40 PSI. The solenoid valve of the CO2 regulator is the powerhouse of the regulator. It is an electromechanical ON / OFF valve that controls the output of carbon dioxide gas into the aquarium.

The CO2 Cylinder – The CO2 Cylinder is a high pressure storage cylinder for the carbon dioxide (CO2) that you will be introducing into the planted aquarium. This is where the complete system starts, inside of the cylinder. Carbon dioxide in a cylinder exists primarily in the form of liquid CO2, only the head space of the cylinder contains gas. The liquid allows the cylinder to maintain a constant high pressure. Because the cylinder contains liquid gas, it must always remain in the upright position.

There are several sizes of CO2 cylinders. The most standard size used for the planted aquarium tends to be the 5 lb cylinder, however, a larger 10, 15, or 20 lb cylinder or a smaller 2.5 lb cylinder can also be used, based on your needs. If your aquarium has space restrictions, then the 2.5 lb cylinder may be ideal for you. On the other hand, with a larger cylinder, you have the potential of saving time and money. The increased storage capacity of a larger cylinder allows you to refill less often and potentially save money on refills. In general, the average cost of refilling a 10 lb cylinder is not much more than that of a 5 lb cylinder, and you get twice the amount of CO2.

pH ControllerThe pH controller provides a full-time, automatic pH monitoring system for the planted tank. It regulates the release of carbon dioxide, which is directly related to pH. The controller is designed to connect to the solenoid of your CO2 regulator. It is set to a desired pH level to be maintained in the aquarium; it then signals the solenoid valve which then prompts the regulator to release or to stop releasing CO2 in order to maintain the set pH.

The controller enables you to maintain consistent and proper CO2 levels. It is an extremely valuable tool in creating a healthy and stable aquatic ecosystem. It will help your plants flourish and can decrease the level of stress to your fish by eliminating fluctuation in pH.

Automatic Timer – The automatic timer provides a more basic approach to CO2 regulation. It allows you to control your regulator and aquarium lights simultaneously and effectively. The solenoid of your regulator can be plugged into one side of a dual outlet timer and the aquarium lights can be plugged into the other side. The timer is then set to turn the regulator and the lights on in the morning at the desired time; promoting an ideal environment for plant photosynthesis. Set the timer to turn off the lights and CO2 in the evening. It’s simple, easy to use, and very useful.

Which to use, a pH Controller or Timer?When using a timer, it is your responsibility to monitor and adjust co2 levels in the aquarium. CO2 levels are monitored through the use of a drop checker, and through observation of the health of your fish and plants. Fine tuned adjustments to CO2 levels are made with the regulator’s needle valve, by adjusting the bubble rate, or the number of bubbles per second entering the aquarium.

Which to use, a pH Controller or Timer?

Using a timer, rather than a pH controller, can be considered a basic and inexpensive method of automating a CO2 system. On the other hand, a pH controller provides a full-time monitoring system of pH levels in the aquarium. It will regulate the release of CO2 in order to maintain a set desired pH, day and night. In comparison, using a timer versus a pH controller may save you a little CO2, because a timer shuts off the flow of CO2 gas at night when CO2 is not necessary.

The Drop CheckerRegardless of whether you decide to use a pH Controller or a timer, it’s always a good idea to use a drop checker to monitor and fine tune CO2 levels. The drop checker is a glass reservoir designed to contain an indicator solution with a known KH (Carbonate Hardness). When submerged, carbon dioxide in the aquarium is absorbed into the indicator solution, until a point of equilibrium is reached between the aquarium water and the solution. As CO2 gas is absorbed into the indicator solution, it lowers the pH of the solution, which in turn changes the solution color. This color, when compared against a pH color chart, allows you to gain an accurate perspective of the concentration of CO2 in the aquarium.

If you use a timer to automate your system, the drop checker is integral; it will be your primary measure of carbon dioxide. If you decide to use a pH controller, the drop checker is an excellent tool in helping you to determine and fine-tune the set point of your controller.

If you are introducing carbon dioxide into the aquarium via a pressurized CO2 system, it is recommended to have a drop checker. It is a good idea to always have an at-a-glance measurement of the CO2 in your aquarium.

The Check ValveThe check valve is simple and essential. It attaches in-line within your CO2 tubing and permits flow in one direction only… into the aquarium. It keeps water from back-siphoning from the aquarium into your vital components, the CO2 regulator. A complete CO2 system is not complete without it.

CO2 Resistant TubingThe pathway through which CO2 travels to the aquarium; it completes the CO2 system, bringing it together. For this reason, it is one of the most important components of the system. It is the job of the tubing to safely deliver your precious CO2 to the aquarium. This is why it is important to invest in CO2 resistant tubing, through which CO2 is not able to escape. Silicone tubing should not be used in the planted aquarium CO2 system; carbon dioxide gas is able to permeate through the walls of silicone tubing, and is wasted. So make sure to use a CO2 resistant tubing so that your aquarium gets what it requires, efficiently, and so money is not wasted on lost CO2.

The tubing connects to the output line cap of the bubble counter, on the regulator, and travels up and into the aquarium. The check valve needs to be placed in-line, within the tubing line, between the regulator and aquarium. For those regulators without a built in bubble counter, an in-line bubble counter can be secured in-line within the tubing line so that you can accurately count the number of CO2 bubbles per second entering the aquarium. One advantage to having an in-line bubble counter is that you can place it above the aquarium stand so the bubble rate can be monitored at a quick glance.

The ‘In-Tank’ CO2 Diffuser – Finally, we have reached the end of the complete CO2 system, the end journey of the CO2 before it is dispersed into the aquarium water. Placed inside the aquarium, at the bottom of the aquarium, the diffuser exists at the end of the tubing line. It is another very important part of the complete system as it transforms and optimizes the CO2 gas entering the aquarium into a usable form of CO2. As CO2 bubbles pass through the porous ceramic of the diffuser, they are diffused into streams of tiny bubbles. With an increased surface area, these tiny bubbles can be readily dissolved into the water, increasing the overall saturation of CO2 in the aquarium ecosystem for efficient plant absorption and less waste.

It is important when selecting a CO2 diffuser to invest in one that will meet the size requirements of your aquarium. Be aware of this when selecting a diffuser for your aquarium, and consider using two diffusers, one on each side of the tank, for those larger tank setups.

A Closer Look at the CO2 Regulator, as it relates to the Planted Aquarium

Carbon Dioxide (CO2) is fundamental to the function and success of the planted aquarium. Carbon dioxide is a chemical compound composed of two oxygen atoms bonded to a single carbon atom (O=C=O).

During photosynthesis, terrestrial and aquatic plants use carbon dioxide and water, removed from the atmosphere and combined with light energy to produce oxygen and sugars. Free oxygen is released as a gas from the decomposition of water molecules (H2O), while the hydrogen is used to generate chemical energy required for the formation of sugars, or glucose. These sugars may then be consumed in respiration or used to produce polysaccharides, complex carbohydrates, such as starch, cellulose, proteins, and other organic compounds required for plant growth and development.

In the planted aquarium ecosystem, carbon dioxide is introduced through the use of a regulator in combination with other specialized CO2 equipment; a complete CO2 system is designed to promote an ideal and balanced ecosystem.

To understand the regulator, we must first take a closer look at the CO2 Cylinder.

CO2 Cylinder – Carbon dioxide (CO2) in a cylinder exists primarily in the form of liquid CO2, only the head space of the tank contains gas. The liquid allows the cylinder to maintain a constant and high pressure; as long as the cylinder contains any amount of the liquid CO2 the regulator’s high pressure gauge will read full, between 800 – 1000 PSI. When the liquid has completely evaporated, CO2 remains in the tank in its gaseous state, and the pressure will decline to zero. Because the cylinder contains liquid gas, it must always remain in the upright position.


CO2 Pressure Regulator – designed to reduce the high pressure inside of a CO2 cylinder to a lower, usable pressure that can be dispensed. The pressure-reducing regulator takes a pressure of 800-1000 PSI (pounds per square inch) from the cylinder, and regulates it, providing a controlled, reduced pressure output in the range of 1-40 PSI. Our CO2 regulators have CGA (Compressed Gas Association) 320 fittings for USA and Canada. A CGA 320 fitting has a 0.825-14 NGO-Right Hand Thread.

High Pressure Gauge – located at the nine o’clock position, it reads the amount of pressure present in the cylinder. As long as there is liquid gas in the cylinder the pressure will read at or around 800 – 1000psi. For CO2 cylinders, once the gauge reads in the red, the liquid gas is depleted and the cylinder should be refilled. The high pressure gauge does not represent the amount of liquid carbon dioxide in the tank. This measurement can only be determined by the weight of the tank less the tare weight (TW) of the tank; this determines the weight of liquid present. The tare weight is printed on the neck of the cylinder.

Low Pressure Gauge – located at the twelve o’clock position, it represents the working pressure or output pressure; the pressure you are using, which can be adjusted.

Adjustment Screw – Adjustments to the output pressure (read on the low pressure gauge) are made with the adjustment screw or t-handle located in the center of the regulator body. The output pressure is set by turning the screw clockwise to increase the output pressure. To reduce the output pressure, the screw should be turned completely counter-clockwise.

Tank Connector and Connector Nut – located at the three o’clock position, it attaches the regulator to the cylinder. Due to high pressure in the cylinder, this connection is a common location for leaks; it is critical to securely fasten the connector nut using the correct seal, nylon or permaseal. The connection must be tightened with a crescent wrench or CO2 wrench.

Solenoid Valve – the solenoid valve is an electromechanical device that controls the flow of CO2 gas from the regulator to the aquarium. The solenoid should be connected to a timer or pH controller to control the input of CO2 into the aquarium. It is the on / off valve. The solenoid is typically mounted on the low pressure side of the regulator. The pressure in the solenoid will not get higher than the set output pressure of the regulator.

Pressure Relief Valve– functions to release excess pressure in the regulator.

Needle Valve – one of the most important parts of the regulator. The needle valve provides precise control over the amount of CO2 allowed to enter the aquarium. It allows you to fine tune and regulate the rate of flow (the number of CO2 bubbles per second) at the desired level. Not all needle valves, however, are created equal. It is important to invest in a quality needle valve for increased precision and to avoid the potentially negative effects of “end of tank dumps”. As the liquid gas in a cylinder nears empty, the pressure in the cylinder will decrease, causing the output pressure to quickly increase and potentially “dump” out of the cylinder into the aquarium. A needle valve will control this “dump” because it functions as the gateway that controls the volume of gas entering the aquarium, it will stabilize the increased output pressure at the set rate of flow (bubbles per second) preventing the CO2 from completely dumping into the aquarium.

Bubble Counter – a visual tool in measuring the number of bubbles per second entering the aquarium. A bubble counter allows you to count the exact number of bubbles per second so that fine adjustments can be made to the rate of flow.

The Drop Checker – How to & the Science behind it

1. Preparation of Indicator Solution:

The correct indicator solution is a combination of KH Standard / 4 degree KH (4 dKH) and pH indicator (Bromothymol Blue).

With drop checker in hand, turn upside down and fill the reservoir of the drop checker approximately half full (or to level manufacturer recommends) with 4 dKH.  A syringe is an ideal tool to use for this.  Then, add approximately 2 – 4 drops of pH indicator (Bromothymol Blue) until a strong, TRANSPARENT blue color is achieved.

The Science:

pH and pH IndicatorpH is a measure of the acidity or alkalinity of a solution. pH indicator is a chemical detector that causes the color of the solution to change depending on pH.

KH and KH StandardKH (Carbonate Hardness) is the measurement in degrees of carbonate and bicarbonate in the water.  KH Standard is a solution with a known KH, a standard, with no other buffers other than carbonate and bicarbonates.

To measure CO2 in the aquarium, you need to have an accurate measurement of KH and pH.  With a known value of KH / carbonate hardness in the indicator solution, the color of the solution will turn green when the optimal concentration of 30 ppm of CO2 is achieved in the aquarium water.  A yellow or blue color indicates too much or too little CO2 in the water, respectively.


2. Place in Aquarium and wait to observe color change:

With the correct side up, place the drop checker in the aquarium, in a visible location.  After approximately one hour, you should notice a change in the color of the indicator solution.  After about two (2) hours you should be able to observe the color and take an accurate measurement of the concentration of CO2 in the aquarium.  For increased accuracy it is recommended to observe the color against a solid white background.

BLUE ->  too little CO2 / low CO2 levels  ->  increase the rate of CO2

GREEN ->  proper CO2 levels  ->  no action required

YELLOW ->  too much CO2 / high CO2 levels  ->  decrease the rate of CO2

The Science:

The drop checker is a reservoir designed to contain an indicator solution and an airspace or air bubble.  When submerged in an aquarium, the airspace eliminates contact between the indicator solution and the aquarium water.

When CO2 is injected into the aquarium, the CO2 will out gas from the aquarium water into the airspace of the drop checker.  As CO2 enters the air, it is absorbed into the indicator solution.  The absorption of CO2 is the result of the gas seeking a point of equilibrium, or equal distribution of CO2 in the solution and the air; the basic principle behind the function of the drop checker.  As CO2 gas is absorbed into the indicator solution, it lowers the pH of the KH standard, which in turn changes the color of the pH indicator.  This condition of equilibrium allows the drop checker, a simple glass reservoir, to provide a highly accurate measurement of carbon dioxide (CO2) in the planted aquarium.

This process is reversed when the injection of CO2 is decreased or zeroed, and the indicator solution contains more CO2 than the aquarium water.  The CO2 will outgas from the indicator solution to seek equilibrium with the airspace and the aquarium water.  This out gassing of CO2 will raise the pH of the solution and change the color of the pH indicator.

In summary, never underestimate the power of the drop checker as an accurate tool for the measurement of CO2 (when used correctly).  Look forward to a video to follow.