CO2 For The Freshwater Aquarium

CO2The use of carbon dioxide in an aquarium is with no doubt the best option for a planted setup. Carbon, being one of the main building blocks of plants, helps promote faster, denser and more vibrant growth. Leaves are fuller, more radiant and brimming with life. Pearling – tiny oxygen bubbles forming on the leaves, is one of the most satisfying things when CO2 is added to the aquarium. These bubbles floating to the surface throughout the tank adds an extra dimension to a freshwater aquarium.  Plants cannot communicate with words but with the introduction of CO2 into the aquarium, you can definitely tell they are happier.

CO2 alone won’t bring about the completion of a well planted tank, but it will definitely make it easier to achieve, allowing a whole new range of plants to choose from. Other factors like light and fertilizer come into play. Add fertilizer the way you would any other tank. Light on the other hand, may need to be changed if you are not achieving rapid growth.

The depth of your tank will determine the strength of light needed for the aquarium. If you are not achieving rapid growth on your foreground plants then your light is not strong enough. On tanks that are over 45cm in depth it is better to use a light source like High Output T5’s, Metal Halides or LED’s specifically designed for planted aquariums.

Only when you have enough light on the aquarium will your plants grow to their full capacity. Light plays a major factor with CO2 as plants use carbon and light to photosynthesis – Converting them into usable sugars that allow the plant to grow. Much like our bodies, where we consume varies nutrients and convert them into the fuel we need. When we eat high nutrition food our bodies perform better. The combination of high light intensity with CO2 enables plants to photosynthesis properly. With more light, plants can absorb more CO2 and fertilizer, in turn helping them to grow faster and stronger. You don’t have grasp the process exactly and there is a lot we don’t even understand about plants, but the process works and we can see the results reflected back to us in the beautiful landscapes that are created.

Carbon Dioxide is present in the water but not enough that plants benefit from, especially in a fully planted aquarium. This is why CO2 needs to be added in gas or liquid form. Liquid CO2 is a great additive to supply more carbon to plants, but it is not enough compared to CO2 in gas form.

Some plants do not need CO2 and we can even grow more difficult plants without it. This is great to some extent but the plant will never grow to its full potential. If adding CO2 to a low light planted tank, the plants will still absorb the carbon but with the reduction in light there will be a reduction in photosynthesis. Even the smallest amount of CO2 will help with plant growth. A low light tank may not get rapid growth, but the plants will be able to make use of this extra carbon and improved growth will result.

CO2 should be turned off at night. Plants do not use carbon when the lights are off so adding this will only make your tank become more acidic. They do the opposite and absorb oxygen from the water in a process called Cellular Respiration – Plants absorb oxygen and expel carbon dioxide. In heavily planted tanks, agitation of the water surface maybe needed for your fish to maintain a sufficient amount of oxygen during the night. This can be from using the outflow pipe to disrupt the water surface or using an air pump. Pay close attention to your fish to determine how much is needed.  The duration will be determined on your tank setup and is needed to offset the cellular respiration process.

By turning on your CO2 a couple of hours before lights go on, you will maximize the use of your CO2.

Types Of CO2

Pressurized Systems are the most reliable and easiest way to inject CO2 into the aquarium. This is because the parts are made specifically to do the job. If using a CO2 system at the very least it needs to  include a CO2 Bottle, Regulator with needle valve, Bubble Counter, Diffuser, One Way Valve and Tubing. Adding a Magnetic solenoid would better serve you as this will give you the ability to turn your system on and off automatically.

Small tanks are ideal for purchasing your own pressurized bottles as the gas lasts for quite a while. With larger tanks it is best to hire a CO2 bottle from a local gas company. This saves time of having to travel to your local fish shop for them to re gas. It may be cheaper or work out the same in the long run, but it is a lot more convenient. Plus you don’t have to fork out money on owning your own pressurized bottle. Before refilling, always freeze your CO2 Bottle as this will allow more gas with each refill.

Disposable CO2 Kits that use throw away cartridges are another alternative. Once the cartridge empties it is then replaced with a new one. These are better used on smaller tanks because they are usually designed for them. Cartridges only come in a couple of sizes making gas usage on bigger tanks expensive.

DIY Do it yourself CO2 is great to try, especially if you have never used CO2 before. Setting up a DIY CO2 system allows you to see the effects of carbon dioxide on the aquarium at virtually no cost. These have to be refilled constantly so the maintenance is high, which can be a bit of a nuisance. Plus there is the added problem of leakage if you haven’t sealed all the parts properly or don’t add the right amount of ingredients. Nowadays CO2 systems are not that expensive as they used to be so it is easily affordable to purchase one of the other alternatives above.


Be sure to use the right diffuser for the size of the aquarium. The diffuser is key to maintaining the plants ability to absorb CO2 in your aquarium water. There are a few different types on the market.

Diffuser Usually made of glass, this is one of the most popular for smaller tanks. It sits directly in the aquarium diffusing bubbles. Unless placed in an area away from the light it will need to be cleaned regularly due to the build up of algae that will reduce its output.

Reactor Can be used externally or internally usually with a separate pump so as not to reduce the flow of your main filter. CO2 is injected in through the top and forced down through plastic balls or other media to break up bubbles into absorbable CO2 for your aquarium plants.

Inline Atomizer Placed inline on the outlet hose of your filter, this remarkable device diffusers CO2 directly into the aquarium through your outlet pipe. Uses Reverse Osmosis technology to reduce the CO2 gas into absorbable bubbles to be used by aquarium plants.

Ways of Measuring CO2

Drop Checker – You can use a drop checker which is made of glass. A pre made solution is mixed with some tank water and it is placed in the aquarium. It will measure the amount of CO2 in the water and change colour accordingly. Too much CO2 the water turns Yellow. Too little turns blue. And green for the right amount. Some people find this to be inaccurate but can help as a quick reference.

PH Controller is the easiest and most accurate, also the most expensive. This device measures the pH of the water at all times and turns the CO2 on and off constantly keeping the pH at the desired level.

Table measuring PH & KH – You can determine the amount of CO2 in the water by measuring your pH and kH then checking against a pre-made table. Between 15 – 30ppm is considered the best range for aquarium plants.

CO2 ChartSomething to be aware of is when measuring your pH and KH the scale is logarithmic. So with a KH of 4 and increments moving from pH6.8 to 6.7 are small increases of CO2 of 3.9ppm but a change from pH6.8 to 6.4 increases CO2 levels to 28.8ppm. And even more dramatically as you reduce your pH even further. From pH6.4 to 6.0 CO2 increase is 72.2ppm. Some readings can be out due to fluctuations in the tank of your KH especially at higher levels.

Even if you are receiving a reading of 30ppm you may need to still increase your CO2 slightly to make sure your aquarium is receiving the desired amount. Always check your fish for signs of stress and make small adjustments over longer periods of time.

Bubble Counter will help with keeping your CO2 constant and is also a quick reference for your CO2 levels. Once you have worked out your desired level of CO2 in the aquarium, count the amount of bubbles you are using per second. Over time your tank parameters may change slightly with your CO2 needing only minor adjustments. Periodic testing will help with this.

When using CO2, first and foremost make sure your fish are well and there are no signs of stress. Small adjustments are better than big ones. If you feel your CO2 levels are not high enough, increase it slowly, come back the next day and check again. The sweet spot is always found with the most minimal of adjustments. If you are not getting the growth you want, then maybe it is time to invest in increased lighting or even new lighting. Any addition of CO2 will help your plants to grow healthier and vibrant. It is definitely noticeable once you begin the addition of CO2. It also opens up the availability to grow a wider variety of aquatic plants, which will help you achieve the aquarium you have been dreaming of.