How to Add Carbon Dioxide (CO2) to Aquarium Plants without Harming Fish?

Carbon dioxide is unquestionably the most important element in a planted aquarium.

Just like plants grown on land, aquatic plants also need carbon dioxide, light, and water to produce energy.

Aquarium plants require a steady supply of carbon dioxide during light hours otherwise their health and growth will suffer.

In this article, we’re going to tell you everything you need to know about adding carbon dioxide to your aquarium.

How to Add Carbon Dioxide to Aquarium Plants without Harming Fish

When plants are growing on the ground, they get ample carbon dioxide from the air, mud, and decomposing plants.

However, in an enclosed aquarium, the amount of carbon dioxide is very limited.

The carbon dioxide that fish produce while respiring is often not enough.

Also, there is minimal plant decay in the aquarium.

Thus, it becomes necessary to supplement carbon dioxide in the aquarium if you want your aquarium plants to grow stronger and better.

However, you also need to consider the other inhabitants of your aquarium – the fish.

Excessive carbon dioxide will not only change the water chemistry but also affect the health of your fish.

Therefore, care needs to be taken when supplementing carbon dioxide into fish tanks.

The latest carbon dioxide dosing units for aquariums have made this task easy and safe.

The Basics of Injecting Carbon Dioxide into Your Fish Aquarium

Whether or not your aquarium requires a carbon dioxide injection and how much you have to inject depends on the type and quantity of plants you wish to grow and also the amount of light you’re supplying.

It’s always recommended to go for an external carbon dioxide supply for a more successful planted aquarium.

For a low-light aquarium, a carbon dioxide injection may not be necessary.

Under low light conditions, plants are less stimulated to grow and the carbon dioxide naturally produced from fish respiration and dead plant matter is sufficient.

Extra carbon dioxide can disturb the water chemistry and will be harmful to fish.

For medium to high light aquariums, a carbon dioxide injection becomes crucial.

With greater light intensity, plants grow faster and their carbon dioxide demand is significantly higher.

In the absence of adequate carbon dioxide, the plants will suffer from stunted growth and it will promote algae formation in the tank that will stress out your fish.

Things You Need to Get Started with Carbon Dioxide Injection

Here is a list of things you need to set up a carbon dioxide injection for your fish aquarium.

Carbon Dioxide Bottle

These bottles come in a variety of sizes storing different volumes of the gas.

The larger the size of the bottle, the longer it’s going to last you. A larger bottle is also more economical to fill up. This cut costs in the long run.

These bottles are readily available for purchase in-store.

Pressure Regulator

Carbon dioxide stored in the bottle isn’t directly pumped into the aquarium.

It’s first introduced into a regulator where the pressure of the gas coming from the bottle is lowered down to a more usable pressure.

It’s best to get an easy to use a single-gauge pressure regulator. It can also change the rate at which the gas is discharged via an additional needle valve.

Aquarium Solenoid Valve

Solenoid valves for aquariums are extremely beneficial as they prevent any wastage of carbon dioxide and save you a lot of money. At night, plants stop photosynthesizing and they no longer need carbon dioxide. This means you have to switch off the gas supply.

While you can do this manually by simply turning off the carbon dioxide supply form the bottle, it’s not always practical to do that and can be quite a hassle. Moreover, you might not even be at home every time the gas supply needs to be turned off.

This is where the solenoid valve comes in handy. Fitted together with a timer, it can shut off the gas supply exactly at the time you want it. You can easily find it from a local store along with a simple 24-hour timer.

CO2 Bubble Counter

This device helps you monitor the rate at which carbon dioxide enters your aquarium. This external piece of equipment should go in line with your filter tubing.

When you’re adjusting the pressure regulator, you can use the bubble counter to see how many bubbles enter the fish tank one after the other.

A good measure of speed is to count the bubbles released every second. If you need to decrease the rate, you can do so by adjusting the regulator.

CO2 Diffuser

This device allows carbon dioxide to enter the aquarium in the most effective manner.

It basically makes the gas push through a porous membrane that breaks down the gas and turns it into a fine mist of bubbles.

It’s easier for your aquarium water to absorb the gas in this way.

You should place the diffuser on the side opposite to the outlet flow. This will then force the gas bubbles downwards the gas will diffuse at a better rate.


A small piece of tubing connects the regulator to the diffuser.

It must be made of a material that’s safe to use with carbon dioxide.

Regular air tubing is not fit for this purpose as it’s resistant to carbon-dioxide-associated damage.

Drop Checker

This piece of equipment is extremely important for monitoring the level of carbon dioxide that has entered the aquarium system.

It is a small vessel that carries a special pH reagent usually bromothymol blue.

As the gas in the water comes in contact with this reagent, it reacts with it and causes its color to change, ranging from dark blue (high pH/ low carbon dioxide) to green and finally to yellow (low pH/ high carbon dioxide).

It’s best to prepare the bromothymol solution using water with a KH of 4°. This way, the levels of carbon dioxide will be monitored more accurately.

It’s best to maintain the carbon dioxide level of 30 ppm which is indicated by a green-colored solution.

Too much carbon dioxide will put your fish at risk of dying and too little will not be enough for successful plant growth.

Aim for a green color and adjust the rate of carbon dioxide using the bubble counter and needle valve.

Turning on the Gas Supply Safely

Once you’ve connected all the components together, you should open the needle valve slightly before releasing the gas from the bottle.

After that, you can turn on the main gas valve to release carbon dioxide.

Gradually turn the needle valve on the regulator to about half a turn and observe the carbon dioxide bubbles through the bubble counter.

You should initially aim for 1-2 bubbles coming out per second and adjust the needle valve accordingly.

Very large aquariums may require a slightly higher rate.

You should also be careful with handling the needle valve as it’s quite sensitive and only small movements are needed to change the rate.

Over the next 24 hours or so, you should keep an eye on the drop checker to monitor your carbon dioxide levels. Keep adjusting as necessary.

This will take some time as the drop checker usually takes at least an hour to start reacting to the levels of carbon dioxide in the water.

Therefore, when you look at the drop checker, its color represents the levels as of an hour ago.

Set the timer for the solenoid valve so that it switches off the gas supply at night.

Some Tips to Follow

Take note of these simple tips that can help you get started with carbon dioxide supplementation for your aquariums.

  • Set the solenoid valve timer for about 1 hour before the lights go out completely. This is because there will be enough carbon dioxide in the fish tank for the plants to utilize during the final minutes of the photoperiod. This will not only save your gas consumption but also keep the carbon dioxide levels in control.
  • For the next day, set the solenoid to turn on the gas supply 1-2 hours before the sun is out. For larger tanks, you might need a longer time period. This ensures that the carbon dioxide levels in your water reach optimum concentration so that the plants start using them quickly when the light period starts. This is also important because the rate of photosynthesis is highest during the beginning of the photoperiod. So, try to get a green color on your drop checker before the sun is out.
  • You have to be careful to not inject too much carbon dioxide into the aquarium. This can be extremely harmful to your fish. One of the best ways to do this is to keep an eye on your fish. Early symptoms of carbon dioxide intoxication include loss of appetite, sluggish behavior, gasping for air, etc. In severe cases, your fish may get unconscious or even die. If you notice such symptoms, it’s recommended that you turn off the gas supply and do a water change as soon as possible.

To Sum It Up

Well-planted aquariums are essential to boost the health and well-being of your aquarium fish.

However, if you wish to grow plants in your aquarium, you have to provide them with an adequate supply of carbon dioxide.

Purchasing good quality equipment to inject carbon dioxide into your aquarium and closely monitoring the gas levels can help you strike the right balance needed to maintain the optimum carbon dioxide concentration in your aquarium.

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