CO2 For Planted Tanks (What It Does & Do You Need It)

If you have a planted aquarium, carbon dioxide (CO2) is probably the single most important factor. Photosynthetic plants in water need it for respiration and growth. During the day, plants need a steady stream of carbon dioxide (CO2) to thrive. They use carbon dioxide along with water and light to create oxygen and carbohydrates for photosynthesis.

Substrate (mud) and decomposing plants are major sources of CO2 for plants in the wild. However, there is a severe lack of carbon dioxide in an enclosed tank. Your tap water lacks CO2, and aquarium plant degradation is modest compared to the natural environment.

Let’s talk about what carbon dioxide does for aquarium plants, as well as some of the ways you can supplement carbon dioxide to your planted tanks.

Do You Need Co2 For A Planted Tank?

In a tank with only fish, the waste products of the fish will produce enough carbon dioxide for plant development; therefore, additional CO2 is not usually needed. However, additional CO2 may be required to keep the plants alive in a planted tank containing fish.

In order to ensure healthy plant growth in a planted tank, it may be necessary to increase the concentration of carbon dioxide in the water. If you have plants in a tank, adding carbon dioxide will help them thrive. Adding carbon dioxide (CO2) to an aquarium can be done in several ways. In either case, keep an eye on the dissolved CO2 levels in your aquarium water to ensure they don’t rise to dangerous levels and kill your fish.

How Much CO2 Does A Planted Tank Need?

Plants, like all other organisms, need carbon, the “building block of life,” in order to survive and thrive. Carbon dioxide, present in the air at several hundred parts per million, is the primary source of carbon for most plants. Fully submerged plants in a planted aquarium would have to get their carbon demands met by the minute amounts of CO2 dissolved in the water. Increasing the carbon dioxide (CO2) level in your aquarium will help your plants grow larger and healthier, but doing so in excess will kill your fish. 

CO2 makes water slightly more acidic when dissolved; when the acidity level rises too high, it can be harmful to fish. Understanding the optimal CO2 concentration for plant growth and fish well-being is essential. Maintain a CO2 level in the tank between 15 and 30 parts per million. Higher than 30 parts per million of carbon dioxide (ppm) can be harmful to fish, thus, aiming for 25 ppm is preferable. Actually, it’s not as hard as you may think. Checking if the pH isn’t lower than 6.2 or using a basic in-tank color monitor are all that is required.

What Does Co2 Do For Plants?

The use of carbon dioxide gas is often cited as the silver bullet that will make plants grow at an astonishing rate while simultaneously wiping out all traces of algae. To better understand the benefits of using CO2 for aquarium plants, let’s examine what it truly does.

CO2 Is Essential for Photosynthesis

For photosynthesis to happen and for plants to make their own food, they need carbon. This is essential regardless of whether or not CO2 gas is being introduced into the aquarium. Plants in a low-tech tank can make use of the 2–3 ppm (parts per million) of CO2 produced through surface gas exchange and animal respiration. CO2 is diffused into a high-tech aquarium to offer extra carbon “food” for the plants, allowing them to develop more quickly. CO2 injection, along with adequate lighting and nutrients, can help aquarium plants thrive and develop swiftly.

Co2 Decreases pH

Slight amounts of carbonic acid (H2CO3) are produced when CO2 is dissolved in water (H2O). The pH of the water in your aquarium will drop after being treated with this weak acid. Animals, such as fish and invertebrates, who live in a planted aquarium with plants also release carbon dioxide when they breathe, which in turn would result in a severe decrease in pH.

CO2 Reduces Algae Bloom

When paired with the right lighting and fertilizer, carbon dioxide (CO2) injection can dramatically increase plant health and development in a CO2-limited aquarium. Algae will have a hard time competing with plants for food and light when those plants are healthy and prospering.

How To Set Up A Co2 System For Planted Tank

It may seem daunting to set up a CO2 system, but doing so is actually rather easy. Though CO2 systems don’t need much care over time, they may need some fine-tuning at first installation. Focusing on the CO2 system installation, you’ll need to gather the materials first before you can get started with the guide’s instructions.

What You Need:

  • CO2 bottle – Carbon dioxide (CO2) bottles can be found in many different sizes. Larger bottles typically have longer shelf lives. In the long run, you can save money by purchasing a larger bottle because it costs less to refill.
  • Regulator – Initial CO2 introduction is accomplished by the use of a CO2 regulator. The pressure from the bottle is reduced by the regulator to a more manageable level.
  • Solenoid – The solenoid reduces costs and prevents CO2 emissions. While your plants are not photosynthesizing during the “dark” hours, they do not require CO2. A solenoid valve and timer will automatically turn off the CO2 supply at the predetermined interval.
  • Bubble Counter – With the help of a bubble counter, you can keep track of how much carbon dioxide is being introduced to your aquarium. It’s a standalone gadget that goes between your filter and the water supply. You may use the bubble counter to keep track of how many bubbles are entering your aquarium while you fine-tune your regulator.
  • Diffuser – With the help of a diffuser, CO2 may be introduced to the aquarium in a safe and efficient manner. An impermeable material is used to compress the CO2 and turn it into a fine mist of bubbles. Aquaria can more readily absorb these bubbles as a result.
  • Tubing – It is required to join the diffuser to the regulator. The tube must be suitable for usage with carbon dioxide. Use CO2-resistant tubing rather than regular airline tubing because it has the necessary qualities.
  • Drop Checker – The amount of carbon dioxide (CO2) diffused into your water can also be measured with a tool called a “drop checker.” It’s a tiny container filled with an indicator liquid that changes color according to the level of carbon dioxide in your aquarium water. Blue means not enough CO2, yellow means too much, and green means just right.

Steps in Setting up your CO2 System:

  1. Attach the tank to the regulator. To begin, turn the main regulator knob counterclockwise and close the valve on your CO2 tank (counterclockwise turn). Be sure the fine-tuning dial is all the way in and locked (clockwise turn). The regulator nut must be securely fastened to the tank, so put the washer in position and give it a good twist. Get out the pliers or the vice grip and snug everything up.
  1. Fill the bubble counter and connect the tube to the device. With the cap off, fill the bubble counter all the way to the clear part. Fasten the lid again (hand tight). It’s possible that mineral oil has made its way into your diffuse, causing it to stop working properly.
  1. Set up the diffuser. Pull out some CO2-proof tubing and attach one end to the top of the bubble counter. Fasten the nut to stop the tube from leaking (hand tight). Make sure there is some slack in the tubing at the other end; if the distance to your aquarium is short or if you are employing a nearby in-line diffuser, this can be shortened. If your diffuser has a cap, unscrew it, and if it doesn’t, you can attach it straight to the outlet.
  1. Activate the tank and regulator valves. To get started, crack open the valve on your CO2 tank. Next, turn the working pressure knob clockwise until the working pressure gauge reaches 20-30 psi for in-tank ceramic diffusers or 30-40 psi for in-line diffusers.
  1. Connect the solenoid and set the bubble counter. Connect your solenoid to a power source, preferably a timer, and make sure it is set to “On” for the desired amount of time. In order to coordinate your CO2 and lighting systems, it is recommended that you use the same timer for both. If your LED is app-controlled, you need only set the CO2 timer so that it activates when the light does.
  1. Examine the Bubbles. Turn your accuracy knob counterclockwise to open it and observe the bubbles as they enter the bubble counter. Turn this dial until you can see individual bubbles well enough to count them. For aquariums less than 20 gallons in capacity, a bubble rate of one per second is adequate. There is no hard and fast rule regarding how many bubbles per second are needed for what size aquarium; instead, you should go by feel.
  1. Make sure there are no water leaks. Use a soapy water mixture to spray around the bubble counter’s regulator/CO2 tank connectors and any other moving parts. If you notice some loose threads, you should quickly secure them.

It’s possible that you’ll need to try out several combinations of bubble size and CO2 on/off duration to achieve the desired effect. This is because various tanks will need varying amounts of CO2 infusion to turn the drop checker green. Injecting excessive amounts of carbon dioxide could kill your fish, so be careful. The fish and plants in your tank will thrive if you start low and gradually increase the level.

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How To Add Co2 To Aquarium Naturally

If biology and science aren’t your thing, adding CO2 to your aquarium can be intimidating. Fortunately, there are non-chemical approaches to supplying your planted aquarium with carbon dioxide. Listed below are a few examples.

A Gas-Infuser-Kit

It’s a great approach to increase your aquarium’s CO2 levels naturally. Carbon dioxide can be introduced to an aquarium without the use of chemicals, but a diffusion system will be needed to do so. If you want to keep your planted aquarium healthy, this is the best option for providing a steady supply of carbon dioxide to the ecosystem.

The Feces of Fishes

In other words, it’s a carbon dioxide generator that doesn’t require any human intervention. Aquarium plants benefit from the natural CO2 supply provided by the nutrient-rich waste of a significant population of fish.

Bacterial Respiration

It boosts the carbon dioxide level in your aquarium over time. However, it does the trick for plants that require very little or no CO2, so it’s worth a shot. This occurs naturally and has nothing to do with you, but rather the aquarium’s ecosystem.

How To Measure Co2 In Aquarium

Plants are a popular addition to aquariums since they not only provide a natural oxygen source but also enhance the room’s aesthetic value. The ability to proactively monitor dissolved carbon dioxide levels in the water is one of the most pressing concerns in these settings. Here are 3 methods you can try to monitor the CO2 level of your aquarium.

Checking the PH Level Before and After CO2 Injection

A little amount of carbonic acid is produced when CO2 gas is dissolved in water, reducing the acidity of the solution (which will cause the pH level to drop). Since the pH of the tank’s water constantly decreases as CO2 levels rise, you may use this as a proxy for the concentration of carbon dioxide (CO2) in the water if you know your injection rate and off-gassing rate.

The Drop Checker Method

Since this technique uses a colorimetric solution, the outcomes will be more reliable. For 24 hours, a glass bubble filled with Bromothymol blue liquid will be submerged in the tank. The solution will become yellow if the concentration of carbon dioxide in the air is too high.

Gauging CO2 Through Plant Growth

In general, the amount of carbon dioxide (CO2) injected into planted tanks should be equal to the needs of the most CO2-sensitive plant being grown. As a result, the most reliable indicator of whether or not a plant is receiving adequate CO2 is to observe its growth form. The obvious drawback to this approach is that it requires experienced aquarists to interpret plant growth form, which is not the case for beginners.

It is very important to determine the amounts of carbon dioxide that are present within your tank, and this activity is one that should be performed on a consistent basis.

How To Choose The Best Co2 Regulators For A Planted Tank

Companies that manufacture similar goods to one another often innovate by adding new features to their versions of the same product or refining existing ones. CO2 regulators are no exception; they can be found in a wide range of sizes and shapes and include a wide variety of settings.

For your system, you must determine what works best. Choosing the right CO2 regulator for your aquarium might be difficult.  Here is a comprehensive guide for beginners:

Either with or Without a Solenoid Valve

Plants in an aquarium only require carbon dioxide (CO2) during photosynthesis. Since CO2 injection into an aquarium is mostly useless if the lights are out, turning them on is a good idea. Without a solenoid, a CO2 regulator requires constant, direct human intervention to shut off. An essential component of any CO2 regulator for a planted tank is the solenoid valve.

Impact of Needle Valves on Uniform Carbon Dioxide Delivery

The CO2 in the aquarium is released at a consistent rate by means of a needle valve. In order to regulate the CO2 bubble flow rate, the needle valve further normalizes the pressure from the cylinder. The lack of this valve could result in the CO2 being poured into the tank at high pressure when the cylinder is nearly empty.

Whether It’s a Single or Two-Stage Pressure Relief

Even though carbon dioxide is a gas at room temperature, a two-stage valve keeps the pressure steady as it flows from the cylinder into the tank. A double-stage CO2 regulator’s pressure-reducing valve allows for finer regulation of the aquarium’s Carbon Dioxide dump. A single-stage pressure relief requires constant monitoring and attention so that the flow rate can be adjusted as needed.

Gauges That Are Both Easy To Read And Reliable

Some equipment isn’t made for laymen, and CO2 regulators are no exception. Thankfully, CO2 regulators are available and come equipped with straightforward gauges. If you have trouble reading small print, look for a CO2 regulator that has a legible font size.

Durability and Quality of Build

In general, CO2 regulators are built to last, but there are certain companies that go above and beyond by creating versions with exceptional durability in order to stay ahead of the competition. The use of a metal such as brass, aluminum, aluminum alloy, stainless steel, or steel alloy in the design of a CO2 regulator is often indicative of its long-term reliability. Know that the cost of more durable designs will be more because of the higher quality of the materials used.


We all have to compare costs to find the best deal on a product. At the end of the day, you can only buy what you can afford. Consider the benefits of a CO2 regulator versus its cost. It’s possible that a less expensive CO2 regulator will be made of inferior materials or have fewer functions.


Although there isn’t a CO2 regulator that is compatible with every cylinder, there are some that work with the vast majority. Select a CO2 regulator that works with the various CO2 cylinders you want to use. Make sure the regulator you choose is compatible with your CO2 gas cylinder before you buy it.

Signs Of Too Much Co2 In Aquarium

Creatures kept in aquariums, such as fish, shrimp, turtles, etc., are adversely affected by high levels of carbon dioxide. Changes in their behavior could be an early warning sign of high carbon dioxide levels in the water if discovered. High amounts of carbon dioxide typically manifest themselves in a number of ways.

Low pH

Since carbonic acid is formed when CO2 levels are high, a very high CO2 level in the aquarium will result in a very low water pH.

Decreased Activity

Fish activity will decrease if the CO2 level in the tank is increased, as some species are sensitive to changes in carbon dioxide levels. Typically energetic fish species will instead separate themselves and spend their time alone in the aquarium.

Change of Position

There is likely insufficient oxygen in the water if carbon dioxide concentrations are high. As a workaround, some aquatic organisms, such as fish, shrimp, and crabs, will make their way to the tank’s top. This is a symptom of CO2 poisoning, which may be widespread.

Erratic Behavior

Fish with high CO2 levels in the tank lose their appetite and become agitated and unpredictable.

These are typical symptoms of elevated levels of carbon dioxide in the water tank. But the responses of other fish species vary. It’s possible then that certain symptoms won’t be the same as others. Because of this, we advise keeping a careful eye on your tank inhabitant’s behavior while you investigate the problem.


How Much Co2 Do You Need For a 20 Gallon Planted Tank?

It is recommended to have one bubble every three seconds for every five gallons of water. That works out to 1 bubble every 3 seconds for a 5-gallon tank. With a 20-gallon aquarium, you should release 4 bubbles every 3 seconds.

How Long To Run Co2 For Heavily Planted Tank?

For a densely planted aquarium, the plants themselves are the primary factor in determining the CO2 runtime. It is crucial to do a study on the individual demands of the plants in your tank because they may have different requirements for carbon dioxide. However, most plants can thrive with only 10–20 ppm CO2.

Does Co2 Reduce Algae In An Aquarium?

Generally, if your aquarium plants are receiving too much light, boosting the CO2 level in the tank can help to reduce algae. Higher CO2 levels have no direct effect on algae. Improved plant health, brought on by the plants’ increased exposure to the CO2 they need for strong growth, has a knock-on effect on algae and the tank’s environment as a whole.

Does CO2 Harm Fish?

 Carbon dioxide hinders the ability of a fish’s blood to transfer oxygen; hence, high concentrations can be harmful. Even if there is a lot of oxygen in the water, fish that live in water with high carbon dioxide levels (>10-12 mg/L for some fish species) can suffocate.


Co2 is an important element for plant growth in a planted tank. It helps to provide the plants with the carbon they need to thrive and grow. The amount of  CO2 your tank needs will depend on the plants you choose and your specific setup, so do your research and experiment to find what works best for you. Adding CO2 can help your plants grow faster and look more lush, so if you’re looking to take your planted tank to the next level, it may be worth considering.