Giancarlo Podio
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Setting up a CO2 system

CO2 is perhaps the most important nutrient in a planted tank. Carbon contents in tank water can vary depending on fish load (respiration), surface agitation (gas exchange) and of course plant uptake. In low light tanks CO2 is not necessary, the speed at which the plants grow is slow enough that most nutrients including CO2 are being introduced into the water faster than the plants are consuming them. Plants have a hard time absorbing CO2 from water, they do it a lot more efficiently from the air which is why many plants will start to grow considerably faster when they reach the surface. Adding CO2 to a low-moderate light tank will speed up growth, improve quality and allow you to grow many plants that would otherwise do poorly without CO2. With higher lighting, CO2 become essential as the tank quickly becomes CO2 limited, leading to growth deficiencies and unwanted algae.

Methods of adding carbon to a tank
Carbon can be added to a tank in a couple of different ways. The most common are pressurized CO2 cylinders, DIY CO2 and liquid forms of organic carbon. Liquid forms of carbon such as Flourish Excel are great for smaller tanks. It requires daily dosing and doesn't seem to have the same effect on all plants. CO2 in gas form is more efficient than liquid carbon so the first two options are the most common. DIY CO2 is a good way to get started into planted aquaria however at some stage most people will want to upgrade to a pressurized setup which provides higher, more constant levels of CO2 without the weekly task of preparing a new mixture as the DIY setup would call for.

What you'll need
First considerations
A couple considerations to be made before you go and buy yourself the required equipment:

- First off you need to know what is available in your area, nothing worse than having a CO2 tank and having no one to fill it for you. Look up fire extinguisher service centers, welding suppliers, beer distributers and home brewing stores.

- Second we need to decide if we are going to own our own tank or trade it in each time for a full tank, like you do with a BBQ gas tank. My preference is to own my own, mostly because I buy new tanks and trust them more than a tank I don't know the history of, not to mention I also have a fire extinguisher service center close by that only does refills, not exchanges.

CO2 bottle
CO2 cylinders are made either in steel or aluminum, either is fine, aluminum is lighter and probably looks a little nicer but that's about it. They come in various sizes from 2.5lb up, the most common being a 5lb tank as this will fit nicely inside a standard tank stand and will last around 8-12 months on say a 55 gallon tank before a refill is needed. Used cylinders can also be of interrest if the price is right, however do check that they have a valid pressure test date stamped on them. All cylinders need to be tested every 5 years. If you have decided to trade your CO2 tank in each time for a full one, you may need to purchase the first full tank at the same place where you will be trading it in each time it's empty. Some stores will take an empty tank that was purchased elsewhere and swap it for a full one, while others will only swap out tanks that were originally purchased from them. So ask before you buy! Your first tank will cost you anywhere between $50-80 and each time you trade it in for a full one around $10-20.
If instead you have decided to own your tank and have it re-filled each time, you can buy one from many different places, is one vendor I've been quite happy with to date:

Source: BeverageFactory CO2 Tanks

A regulator simply reduces the pressure of the gas from the bottle to usable amounts. There is not much to be said about regulators, you have hundreds of models to choose from. For the budget minded or pure DIY person a standard regulator such as this one will suffice:

Source: BeverageFactory CO2 Regulators

For those looking for the least amount of work and DIY tasks, a complete regulator may be more interesting. Those interested in using PH controllers or want to turn off CO2 at night, these are your best solution. These regulators come with needle valve, check valve, solenoid and bubble counter:

Source: JBJ Regulators on eBay

Needle Valve
A needle valve allows minute adjustments to be made to the amount of CO2 going into the tank. Considering we are talking 30-60 bubbles per minute on an average tank, it's obvious that we need an accurate needle valve that will provide a stable rate. I have been very satisfied using needle valves from Clippard:

Source: Clippard part number MNV-4K2

Clippard 1/4" male to 10-32 female adapter, part number 4CQF-PKG (required to screw the needle valve onto the regulator)

Source: Clippard part number 4CQF-PKG (old SKU: 15006-3)

Check Valve
A check valve stops water from flowing backwards from the aquarium to the CO2 tank. Although this will not happen while the CO2 is being erogated, it can happen when the CO2 runs out or something is disconnected during maintenance. Seeing you don't want to flood the house or send water into the regulator, a check valve is a must in my opinion. You can use a regular plastic check valve used for air pumps, this should be changed at least once a year as the CO2 gas will damage it. Otherwise my personal choice is the Clippard MCV-1 brass check valve. If you're buying the needle valve above you shouldn't even think twice about adding this to your order, it's only a couple dollars more and far better than the plastic models.

Source: Clippard part number MCV-1

Source: Regular airline check valve

Bubble Counter
A bubble counter allows you to monitor the rate at which CO2 is being erogated into the tank. While it provides little to no hint as to the actual concentration in the tank itself (more on this later), it does allow you to adjust the needle valve quickly after maintenance or to make minor adjustments to the bubbles per minute that are dosed. A bubble counter, if not purchased with a complete regulator shown above can be added inline to any setup. Here's an example:

Source: Dennerle Bubble Counter

Alternatively, you can also choose not to use one. I use a valve manifold on all my CO2 tanks and one feed goes to a short tube I push under water to check the bubble rates when needed.

CO2 proof tubing
You'll need tubing to bring the CO2 into the tank. I use regular soft silicon airline tubing as shown below. Although not 100% CO2 proof, it holds up well over 2 years and is cheap enough to replace once a year:

Source: Soft silicon airline tubing
Source: CO2 proof tubing

CO2 Diffuser/Reactor
A diffuser is something that diffuses the gas in the tank, similar to regular air stones... In short, the gas is pushed through a porus medium that releases it in the tank as a fine mist of CO2 bubbles, the finer the better. These bubbles are partially absorbed into the water molecules as they make their way up to the surface. Obviously any bubbles that do reach the surface are usually lost to the atmosphere and so it's safe to say that a diffuser is not as efficient as a reactor. A diffuser should be made of ceramic, glass or other CO2 proof material. Regular air stones are not going to work as the CO2 breaks the bond of the glue holding them together. On the low end there are the ceramic Micro Bubblers from Rena, otherwise a glass diffuser as shown below:

A reactor on the other hand usually consists of a chamber where water is pushed downwards through the chamber and CO2 bubbles are diffused from the bottom. As the bubbles try to float to the top, the water flowing down keeps them trapped in the reaction chamber until completely absorbed by the water. Little to no CO2 is wasted using this method however it is a more complex and expensive method of diffusion, of most interest to those with tanks around 55 gallons and bigger. Reactors can be placed inside the tank (an eye-sore) or externally in a closed loop or inline with a canister filter.

Source: CO2 Accessories
DIY: DIY External Reactor
DIY: Converted gravel vacum

If you have a canister filter you have one more option. That is to send the CO2 directly into the intake tube and use the canister filter itself as a reaction chamber. Initially I was a little hesitant in doing this however since I tried it I have not looked back since. I find this to be the best compromise between esthetics, simplicity and efficiency. It requires no added equipment, it's next to invisible and is just as effective as a reactor. You obviously want to make sure your canister expels trapped air/gas easily and will not "airlock". I use Eheim Classic series canisters in all my tanks and have never had one airlock due to CO2 or other trapped gasses in the canister.

Other interesting products exist such as combination units and complete CO2 startup kits.

Putting it all together
This is the simple part :-) Let's start off with placing the regulator on the bottle and assuring we have a good seal. The regulator should have come with a plastic washer that is used between the regulator and tank to make a good seal, no teflon tape should be used on this side of the regulator as the seal is made by the plastic washer and not the threads themselves.

To test the seal, close the regulator by turning the adjustment screw counter-clockwise until it turns freely. Open the tap on the tank, you should see the high pressure gauge indicate the pressure inside the tank. Now close the valve on the tank and wait about 5 minutes. If no leaks are present, the pressure inside the regulator should not drop even with the tap on the bottle turned off. If you encounter problems and can't get it to make a good seal, use a brush to "paint" some soapy water around the connections to find the location of the leak.

Next is the needle valve assembly for those that purchased a standard regulator. If you purchased a Clippard check valve you will want to place that between the needle valve and adapter.

Use teflon tape to screw the adapter into the regulator, be careful not to allow any pieces of tape to enter the regulator housing or possibly make their way to the needle valve. The rest of the Clippard fittings use an o-ring to make a good seal and can be hand tightened. We'll only be using pressures around 10psi so making a good seal on this side of the regulator is rather easy.

Connect the tubing to bring the CO2 into the tank and connect it to whatever method of diffusion you have chosen to use. If you are using a regular inline check valve make sure you connect it somewhere between the needle valve and diffuser/reactor. If you are also using an inline bubble counter, place the check valve between the bubble counter and the diffuser/reactor.

Setting the pressure
Close the regulator by turning the adjustment screw counter-clockwise until it turns freely. Open the needle valve a couple turns, this is to avoid damaging the valve in case too much pressure is sent to it by mistake. Open the tank valve. No CO2 should be coming out however the high pressure gauge should indicate the pressure inside the tank (Around 800psi when full). Close the needle valve (never overtighten) and slowly turn the regulator's adjustment screw clockwise until the low pressure gauge reads 10psi.

Open the needle valve to test the unit, some diffusers require a little time for pressure to build up and start diffusing. It's impossible to say how much CO2 is required for any given tank, best thing to do is to start low and monitor the CO2 concentration using the KH/PH Chart. A conservative starting point would be a bubble every 3-4 seconds. Aim for an initial CO2 concentration of around 15-20ppm, you can figure out exactly how much you want later based on your own tank.

Some useful information regarding measuring and controlling CO2 levels and avoiding PH crashes can be found towards the end of the DIY CO2 article.

Giancarlo Podio
Original: 03/28/2005
Revised: 05/01/2008

The products I have chosen to show in this article are indicative of those I've successfully used in the past or products I know to be acceptable for the task. This article has been edited and reproduced by third parties to promote their own products and redirect links to their own store (Example). Please note that these versions were edited without my consent and may contain information, links and depict products which I cannot speak for or endorse.