As mentioned in the “Lighting for planted aquarium” article, plant use light source to convert Carbon Dioxide (CO2) and water (H2O) into Glucose/energy (food):

CO2 + H2O + Sunlight energy Glucose + O2

Therefore, without CO2, plants will not be able to carry out photosynthesis and produce ‘food”.  The relationship between CO2 and lighting in planted tank is inseparable as both are the essential element needed for plants growth.

How can we provide CO2 to a planted aquarium?

In nature, CO2 can be found in water (river, stream, lake) through:

1. Respiratory process of the fishes or micro organism in the ecosystem – fishes or micro organism use O2 in the respiration process and releases CO2 into the water
2. CO2 that dissolve from the atmospheric air through the movement of water. CO2 is dissolvable in water so if atmospheric air (which contains 0.04% of CO2 or 400ppm) is in contact with water, some percentage of CO2 can be dissolved.

However in a planted aquarium, CO2 provided by the respiratory process of the fishes and the natural exchange of CO2 between the aquarium water and the air is insufficient, therefore, we need to provide CO2 to our planted tank to maintain at a healthy level for the plants. There are basically 5 main categories of method which can be applied for this purpose:

1. Direct CO2 injection into the tank – a straightforward and most common used method. CO2 injection can be done in 2 ways: from a pressurized CO2 tank or from a DIY CO2 kit.
2. Chemical additives in liquid form – does not really provide “CO2” but carbon. 
3. CO2 produce from solid tablet – the least effective method. 
4. Provide CO2 from electrolysis – generate CO2 using electrode.
5. Certain species of plants can convert and absorb CO2 from carbonate/bicarbonate form. However this method will cause the fluctuation of pH due to precipitation of calcium carbonate as a side effect of this process.

Direct CO2 injection using pressurized CO2 tank is the most recommended and cost effective method.

Measuring CO2 in planted tank

The correct way to measure CO2 is in ppm (part per million), a common guideline would be 25-30ppm for a planted tank. However, not many hobbyist/aquarist are equipped with the knowledge and proper tool for this measurement. There are different ways of measuring CO2 in the tank

1. kH/pH table. This table only work if sure 100% that there is nothing in the tank that alter the ph or kH beside CO2. This is impossible in aquascaping since we use wood that will reduce pH, we use sand and rock that may increase kH.

2. Drop Checker. It will not tell you the exact CO2 content in the tank, but will indicate whether it is too much, too little or just enough, by different color shade display on a special solution you put in the planted tank in a special device called “drop checker”

Blue     : not enough CO2
Green   : enough CO2
Yellow : too much CO2

3. Bubbles per second is a least accurate way but most widely use method to “estimate” CO2 level in a tank and it is only applicable to direct CO2 injection method through a bubble counter. With this method, we estimate how many bubbles of CO2 are being release from the CO2 tank into the bubble counter and use that as a guide to estimate the CO2 level in the tank. (Note: Bps is just an indicative method to let us know how much CO2 is being injected into the tank but it does not tell us the CO2 level in the tank).

Diffusing CO2 in planted tank

There is a common misunderstanding among planted tank hobbyist when it comes to injection of CO2 into the tank. A lot of people would claim that they have “injected a lot of CO2 in their tank” and assume that the CO2 level must be already sufficient. However, they do not realize that the more important factor is not how much CO2 has been injected into the tank, but how well the CO2 disperses and dissolves in the tank. Therefore, even if one have very high bps, but the dispersion and dissolve rate is poor, it is just wasting CO2 because most of the CO2 will rise to the water surface and will not be used by the plants. In order to maximize the dissolve rate of the Co2 injected

• Use better quality CO2 diffuser that produces fine (tiny) bubbles (small bubble = bigger surface area = high dissolve rate)
• Position the CO2 diffuser where the bubbles can be carried by the filter outflow and distribute to bigger areas of the tank.
• Position the CO2 diffuser at the lower half of the tank to increase the distance those CO2 bubbles need to travel to water surface (longer contact time = higher dissolve rate)

The main idea is to increase the contact time and contact area between CO2 bubbles and the water. Some use a CO2 reactor to dissolve CO2 in the water first before introducing them to the tank, which seems to be the most effective way. One debatable method is positioning the CO2 diffuser near the inlet of a filter, some believe this to be a very effective way to dissolve CO2 through the filter media, but some have the opinion that it may cause problem to the filter in long term. In General, there are couples of way to mix CO2 with tank water. The equipments below are commonly available at LFS

• Flipper. CO2 bubble roll along the ladder allowing sufficient time for CO2 to dissolve. Good for DIY/Yeast CO2. Example:

• Internal diffuser. Example:

• External diffuser (inline diffuser). Example:

• Internal reactor. Example:

• External reactor. Example:

• Membrane reactor. The Membrane reactor contains a gas permeable membrane tube which is impermeable to water. When connected to a CO2 source, the gas diffuses through the membrane into the water - free of bubbles and without losses. Example:

When should we supply CO2 in a planted tank?

Plants only need CO2 during their photosynthesis period. Therefore, it is a common practice to supply CO2 only when the light is switched on (8-10 hours a day). Some planted tank hobbyist provide CO2 24/7 to their tanks, it is generally believe to cause no harm to the flora and fauna of the tank but the usefulness and cost effectiveness of this continuous supply of CO2 is debatable

There are also some aquarist who switch on the CO2 1-2 hours before the light is on, it is believed that CO2 should be supplied slightly in advance of the light to “prepare” the water with enough CO2 so that the plant can readily start the photosynthesis process immediate when the light is on. However, there is also theory that CO2 can be switched on 1-2 hours after the light is on, because the first 1-2 hour after light on is the period to “wake up” the plant before they start their photosynthesis. Similarly, some switch off the CO2 earlier than the lighting period and some do it the opposite, with their own beliefs and theories

In general, from our very personal experience, as long as the CO2 supply is about the same time as the lighting period, whether it is switch on/off   1-2 hours before or after the light, it does not have great effect to the well being of the plants. However, if the cost of refilling CO2 tank is not an issue, it is good to supply a little bit more to make sure CO2 is always sufficient for the plant. One thing to take note is that, over supplying the CO2 may cause suffocation to the fauna in your tank, so do it in moderation

As a recommendation, it is good to supply enough or slightly more CO2 to your tank and let the tank tells you how you should go from there. If your fauna are doing well, plants are healthy and algae is under control, it means you are doing the right thing, otherwise, reduce or increase the CO2 dosage accordingly to tackle the problem. Taking care of a planted tank is not an exact science so, after we understand the basic requirement, we should always be prepared to make changes according to the situation

Can planted tank survive without any additional CO2 supply?

Before we answer this question, we have to understand that ALL plants need CO2/carbon to survive. Without CO2/carbon, ALL plants will die.

It is safe to say that all water source contain a certain amount of CO2 because CO2 is dissolvable in water, so even if we do not provide any added CO2 supply to the tank, all tanks should have a certain amount of CO2 in the water, either through the CO2 that dissolve from the water surface, respiratory system of the fauna in the tank or during the water change when fresh water (with CO2) are added into the tank. However, these CO2 are in limited concentration and it will not be able to sustain the requirement if the planted tank has a lot of plants or have certain plant which has higher requirement for CO2.

Therefore, whether a planted tank can survive without additional CO2 supply, it depends. If the plant species in the tank are those like moss, fern, anubias or other slow growing plants (slow growing = lower requirement for CO2), then yes, these plants can survive and grow in a tank without additional CO2 supply. But they will grow at a slower rate and the healthiness of the plant is not guaranteed although they might not die. Other types of plants like most of the stem plants, HC and other more demanding plants may also survive but the chances of them prospering in a tank without additional CO2 supply is slim