Carbon Dioxide for Planted Aquaria

There seems to be increased interest from many fish keepers in planted tanks. The spectacle of lush green plants with our fish swimming in and amongst them is a goal that many of us come to have after fish keeping for a time. But for many that try it turns out to be an experiment gone wrong. One of the key elements in growing plants successfully is the addition of a CO2 system. CO2 can be the difference between boring and spectacular when it comes to your plants. My goal here is to explain the components required for a well balanced planted CO2 fed aquarium. Many people fear you need to be a rocket scientist to be able to maintain such a system but the reality is that could not be further from the truth. If you have a basic understanding of water chemistry it is very simple to understand and manage. There is an investment financially however so you need to understand to have a dependable system you will have to spend a bit of cash.


So to begin the first thing we require is a source of CO2. Some elect to use the very inexpensive yeast fermentation method. You can simply add yeast to a sugar mixture in plastic 2L pop bottles and it will start to produce CO2. This method is in my opinion very risky, messy, and completely unreliable. If you’re serious about your fish and plants the way to go is to purchase a CO2 tank. These can be obtained used from Fire Extinguisher service outlets usually for a third of the price of a new one.

These outlets will usually refill them when required for 15 to 25 bucks depending on the size. They are pressure tested and recertified for use prior to the sale and are perfect for our needs. You can get 5, 10, 15, and 20LB tanks and the size of your tank and available space will dictate what size bottle you’ll want. I use a 15LB bottle on my 130G and I need to have it refilled once a year. It fits neatly inside the stand.

We now need a way to regulate the pressure and control the flow of the CO2. There are many different types available but what I chose to use are the Dual meter types sold for making beer. Our requirements are to have a meter to show tank pressure, another to show the working pressure we elect to use and finally a way to control the flow. Typically we want a working pressure of 1 to 1.5 bar or 15-22 lbs pressure.

Some people now take the CO2 and run it straight into a diffuser or reactor and have the CO2 being fed into the tank 24/7. This is not safe for your fish and requires constant monitoring of your PH as too much CO2 can and will cause your PH to crash. My chosen and the safest method is to utilize a PH monitor/controller. This unit has a probe that goes into the tank water and senses the PH level in the tank. You determine both minimum and maximum PH levels you want to have typically dictated by the fish species and/or plants you are keeping. 

This is where a basic understanding of water chemistry comes into play. I’ll not get into Chemistry 101 but CO2 being fed into an aquarium will cause the PH to drop. You need to buffer against this. It is the balance of Carbonate hardness (KH) vs. CO2 that determines your ph. I’m sure some of you are thinking forget it…this is too hard. We’ll rest assured it’s not. If you use a PH controller it does all the work and you just enjoy the results. The PH monitor controls a solenoid (like a CO2 switch) that turns the CO2 on or off as needed.


As the CO2 is fed into the tank the PH starts to fall. Once it reaches the minimum PH level you have selected it will turn off the CO2. The PH will start to slowly bounce back and once it reaches the maximum PH level you selected it will turn on the CO2 again until it reaches the minimum level and will continuously cycle. The only thing you need to be concerned with is the amount of buffer you add to the tank as the PH controller takes car of the rest. I use simple Sodium Bicarbonate (baking soda) as my buffer of choice. I tsp to 20G’s results in a KH of around 5 degrees. Below is a chart that will help you determine your desired Carbonate hardness (KH) vs. your CO2 levels in mg/l.


This looks way more confusing than it is. Say for example you’re keeping Discus and you want to maintain a PH of 6.75 and you want a CO2 level of 23PPM (you do not want to go higher than 30 or you could stress your fish…23 mg/l is a decent amount). Look along the bottom of the chart and select your desired PH. Go straight up to the CO2 level of 23. Now go straight across to the left hand side and you’ll see you will need a KH of 4. Some of you will be limited as your minimum PH and KH levels straight out of the tap could be higher than this so if that’s the case I’d reccomend you use RO. So lets say you have a KH of 6 out of the tap and aged. You can see by looking at the chart that a PH of 6.75 and a KH of 6 will give you 33PPM of CO2 which is too much and will harm your fish so there is a bit of a balancing act but it is really not difficult. The water I have to deal with is ideal as we have almost no KH or GH out of the tap so I have the luxury of being able to control my KH.

Enough of the Chemistry Class. From here we need to get the CO2 into the tank. I like to have a bubble counter in between the Solenoid and the next topic of discussion the Reactor or Diffuser. A bubble counter is not necessary when using a PH controller but I like to be able to see at a glance that CO2 is flowing when it’s supposed to be so it gives me a very quick visual.

The CO2 now needs to be absorbed by the tank water. To do this we need to keep the CO2 in contact with the water as long as possible so we need a method of doing this. The most common and most efficient are CO2 reactors. They are very easy to make or can be purchased ready to go.

There are also diffusers which are simply a very fine air stone that takes the CO2 and diffuses it into very tiny bubbles that get absorbed by the water but they are not as efficient. There are other methods as well but we’ll focus on the Reactor. A reactor is simply a tube usually containing BioBalls or other media which are there to help the CO2 get diffused in the water. They help increase the time the CO2 is suspended in the water so that the absorbtion rate is higher. I use the AquaMedic Reactor 1000’s as they are efficient and not terribly expensive and is seen above. They can be installed inline on the output of a Canister filter or you can run a small separate water pump as well. They can also be hidden inside the tank behind all those gorgeous Lush green plants!!!