Aquarium Decoration

The best CO2 diffusers and reactors for your aquarium

If you want to have a lively and densely planted freshwater aquarium, you need to fertilize your plants with carbon. Carbon is not easily supplemented in liquid or solid form like other fertilizers and is usually delivered as a gas. To mix carbon dioxide gas into your aquarium water efficiently, you will need the best CO2 diffuser.

Introduction To CO2 Diffusers For Aquariums

Carbon is an essential element for animals and plants. Plants use carbon dioxide (CO2) gas as part of the process of converting light into food for growth and reproduction, known as photosynthesis. Without getting too deep into the chemistry, let’s explore this process for a second.

Land plants extract CO2 from the atmosphere and break the bonds between carbon and two oxygen molecules. As you may remember from your high school biology lessons, plants then use the carbon to make sugars (plant food) and release the oxygen into the atmosphere. But what about aquatic plants?

How do underwater plants obtain carbon?

Outside, the constant cycle of evaporation and rain mixes atmospheric CO2 into bodies of water. Fine bubbles of CO2 are suspended in the water column and only slowly dissipate into the atmosphere. Underwater plants are specially adapted to extract this dissolved CO2 from the water and use it for photosynthesis.

However, this cycle presents a problem for plant-loving aquarists. How do you get CO2 gas to dissolve in your aquarium water? CO2 injection systems and low pressure DIY CO2 kits can easily produce the gas, but you’ll still need a way to suspend the gas in your aquarium water. That’s where a good CO2 diffuser comes into play.

What is a CO2 diffuser and how does it work?

CO2 diffusers take the gas produced by your injection system and mix it with water to form fine, microscopic CO2 bubbles. As the bubbles float around in your aquarium, the CO2 dissolves into the water column, where it can be absorbed and used by your plants. The speed at which this happens is directly related to the size of the diffused bubbles.

Small bubbles have more surface area in proportion to their volume than large bubbles, so finer bubbles allow more CO2 to dissolve in the water in the same amount of time and are more efficient. Large bubbles often break the surface before the CO2 can be fully absorbed, reducing the efficiency of your system.

Depending on their design, CO2 diffusers:

  • Mix CO2 and water to form a mist of microbubbles, which are released into the water to circulate around your tank (reactor and some in-line types).
  • Or they pass the large CO2 bubbles through a fine screen or membrane to produce a stream of fine bubbles before they are released into the water column (ceramic, some inline diffusers and air stone diffusers).
  • Some diffusers also increase the amount of time the bubbles are in contact with the water, which improves the rate of absorption and saturation of CO2 in your tank (reactor, ceramic and ladder diffusers).

Do you need a CO2 diffuser?

If you are using a carbon injection system for your planted tank, you definitely need a CO2 diffuser to get the most out of it. If you place the injection tube in your aquarium without a diffuser, the large bubbles will float up and release CO2 into the air instead of the water. Diffusers are the solution to the big bubble problem!

Benefits of using a CO2 diffuser

The main reason to use a CO2 diffuser is to make your carbon generation system more efficient. Without a diffuser to control the size of the bubbles that are released into your tank, your injection system can waste CO2 and cannot deliver enough dissolved carbon to meet the needs of your plants.

Types of CO2 Diffusers

The exact method a CO2 diffuser uses to break large bubbles into small bubbles or mix CO2 and water varies depending on its design. Let’s take a look at the most common types of CO2 diffusers for home aquariums:

CO2 reactors

CO2 Reactors are the premium way to deliver carbon to your planted tank. These automatic mixers mix the water from your tank and the CO2 from your injection system until almost all of the CO2 is dissolved, resulting in the highest efficiency of any type of diffuser.

However, they are also the most expensive type of diffuser and are not suitable for use with most DIY and low pressure CO2 generators. You will also need a powerful filter or pump to power the blades. They’re not always the easiest to install, but if you want the most efficient diffuser, a CO2 reactor might be the way to go.

Ceramic CO2 diffuser

Some of the most attractive and cost effective diffusers are those that use a ceramic disc or a thin ceramic membrane to create small bubbles of CO2. They are often made of clear glass or stainless steel for easy cleaning and have a ceramic disc or layer in the middle through which the CO2 passes.

This is a very broad category, and installation and function can vary greatly depending on your design. Some ceramic diffusers sit inside your aquarium, while others can be connected in-line to your filter or hung externally on your tank. They are usually not very expensive, which is another plus.

CO2 diffuser online

In-line diffusers hook directly into the vessel’s filtration system near where the water returns through the outflow. This means that they can usually be hidden behind or under your tank and reduce clutter within your aquarium. They are more efficient than ceramic diffusers but not as effective as CO2 reactors.

In-line diffusers are more difficult to install and regulate as they rely on your filter’s motor to push water through the diffuser. They also don’t work with HOBs or internal filters. It is much more common to see them in large tanks with powerful canister systems.

Staircase CO2 diffuser

A ladder-type CO2 diffuser is a simple, inexpensive glass or plastic device that extends the amount of time bubbles are in contact with the water before being released into your tank. They increase the efficiency of DIY and low pressure CO2 generators and are the ideal choice for nano tanks and small planted aquariums.

airstone diffuser

Airstone CO2 diffusers are the same thick stones used to increase the oxygenation of water in a tank, but they send out bubbles of CO2 instead of oxygen. They are also not efficient or very effective. Personally, I’d save my money and invest in a more useful system instead of using an air stone.

CO2 diffuser type Benefits Disadvantages
  • CO2 diffuser of the most efficient type
  • Mixes 98-100% of the CO2 generated in your water
  • Can be located next to or under the tank to reduce clutter
  • The best type of CO2 diffuser for large aquariums
  • Expensive
  • Requires a pressurized CO2 injection system
  • You need a powerful filter or pump to drive the cutting blades.
ceramic diffuser
  • The most common type of CO2 diffuser for nano tanks and small aquariums
  • It uses a ceramic disk or a fine ceramic membrane to break the CO2 into small bubbles.
  • It can be made of glass or stainless steel.
  • Generally inexpensive and available in a wide variety of sizes and designs.
  • Specific benefits vary by design
  • Ceramic quality varies and not all ceramic diffusers work equally well
  • Ceramic is difficult to clean and clogs easily.
  • May require regular replacement
  • Not as efficient as an in-line reactor or diffusers
  • The materials are often brittle and break easily during maintenance.
online diffuser
  • Clips onto your canister’s filtration system near the outlet
  • Can be hidden behind or under your tank to reduce clutter
  • Increases CO2 saturation by extending the amount of time the bubbles are in contact with the water.
  • More difficult to install than a reactor or ceramic diffuser
  • Requires a powerful canister filter or separate pump to push water through
  • If the system leaks or water flows into the diffuser, you could have a costly and complicated cleanup.
stair diffuser
  • Inexpensive glass or plastic tubing that prolongs the amount of time the CO2 bubbles are in contact with the water before being released into the tank.
  • Ideal choice for nano tanks and small aquariums with low pressure or DIY CO2 generators
  • Not as efficient as reactors, ceramic diffusers, or in-line diffusers
airstone diffuser
  • Just like an oxygenating air stone, except the CO2 is bubbled through
  • The cheapest type of diffuser
  • Less efficient and desirable diffuser type
  • Not recommended for any type of aquarium.

How To Choose Your CO2 Diffuser

It is not as easy to convey recommendations for CO2 diffusers as it is for other types of equipment such as filters, LED lights and substrate. The best diffuser for your tank varies depending on the type of carbon injection and filtration system you are using. You will definitely have to do some research to find the best option for your tank.

Let’s take a look at some of the things to consider when choosing a diffuser:

Ideal diffuser type for your setup

There are three parts to a carbon injection system for a planted tank: a CO2 generator, a CO2 regulator, and a CO2 diffuser. The ideal diffuser for your tank will depend on the type of generator and regulator you are using. If you are using pressurized CO2 cartridges, you will need a diffuser designed for high pressure CO2.

Your type of filtration system is also important for many types of CO2 diffusers. Most inline diffusers need a specific amount of flow to push the CO2 through the disc or membrane to form small bubbles. If you’re using it with the wrong type of pump, you won’t get bubbles.

Tank size and flow

Size matters a bit when it comes to CO2 diffusers, but choosing a diffuser that is too small rather than one that is larger than necessary is more of an issue. A small diffuser will be limited by the maximum amount of CO2 it can handle in bubbles per minute. It is not very efficient to use a small diffuser in a large tank.

More than the size of your tank, the location of the CO2 diffuser is what really counts. You want to maximize the amount of time the bubbles spend in the water before they break to the surface. Avoid placing it near the top or next to your filter inlets, and check the flow pattern in your tank to find the best spot to maximize circulation.

Type and materials

Like everything in your tank, CO2 diffusers need to be cleaned and maintained regularly. Minerals in the water can clog the ceramic discs and membranes used to generate the bubbles, and algae often build up on these devices. Look for diffusers that are easy to remove for cleaning and also easy to clean.

While glass is the most common material for CO2 diffusers, stainless steel might be a better choice if you’re particularly clumsy. They’re not as attractive, but they don’t break as easily either. Diffusers made from glass-like resins are also stronger and more durable. Reactors typically come with acrylic or stainless steel chambers.

Kit Inclusions

Sometimes it’s easier to buy a diffuser as part of a kit. You can spend a lot of change on a complete CO2 system that includes a generator, regulator, and diffuser, or you can look for a cheaper diffuser-only kit if that’s all you need to replace or upgrade.

Some diffusers come with bubble counters or glass/resin U-bends in addition to the diffuser. If you need the extra parts, it might be a good deal and save you some money if you get a diffuser kit. You can also buy your CO2 diffuser individually if you prefer.

How to Install Your CO2 Diffuser

There are so many diffusers on the market that it is difficult to generalize about the installation process. CO2 diffusers are usually fairly easy to install, although specific instructions will vary depending on their type and how they connect to your CO2 generator, regulator, and filtration system.

However, I can give you some tips on setting up your diffuser:

  • First, always soak your diffuser in your tank for a while before installing it.
    • The exact duration varies by manufacturer, but it usually works from 30 minutes to 24 hours.
    • This allows the ceramic membrane to fill completely with water, helps push air out of the system, and also fills the diffuser.
    • Diffusers don’t work when dry, so the soaking step is critical.
  • Make sure you have added a check valve between your diffuser and the generator or regulator to prevent water from flowing back into the generator when your system is off (for example, at night).
  • Use the type of tubing recommended for your diffuser.
    • Be sure to seal any loose fittings with plumber’s tape or silicone to prevent pressure loss or system leaks.
    • Pressurized CO2 systems may require barb connections to prevent tubing from backing out when the system is turned on.

Top 5 Aquarium CO2 Diffuser Reviews

There are hundreds of diffusers on the market and many look almost identical to one another. With so many options, it can be challenging to find the right CO2 diffuser for your planted tank. The following diffusers are some of the best, so let’s take a look and see which one would be the ideal choice for your aquarium.

1. fluval ceramic CO2 diffuser

  • Type of diffuser: Ceramic
  • Tank size: less than 30 gallons
  • Dimensions: 3.5 inches long

If you are using the fluval brand pressurized CO2 system in a small aquarium under 30 gallons in capacity, then the ideal replacement would be this small Fluval ceramic diffuser. With its wide ceramic disc, this diffuser produces a constant stream of fine bubbles. It’s easy to install, and while it’s not the most attractive device, it gets the job done. It’s also a very cheap replacement part, which is helpful.

It is perfect for fluval systems and will work with most pressurized CO2 generators as long as they produce at least 30 psi (pounds per square inch). This is a very simple diffuser and it comes with a bit of protection for the ceramic disc and a suction cup to hold it in place.

However, it is not my favorite diffuser, even for fluval CO2 systems. The diffuser is not very easy to clean and the ceramic disc is not removable from the housing. While you can soak everything in bleach or hydrogen peroxide to remove any stains or algae, the connections seem to wear out quickly and this diffuser needs frequent replacement.

Fluval is harder to break than glass diffusers, so for some users, it could be a more durable and cost-effective option. Even if you have to replace it more often due to its marginal quality, you probably won’t break it when you remove it for maintenance.


  • Ideal for Fluval brand pressurized CO2 systems
  • The single ceramic plate produces a fine stream of bubbles.
  • The suction cup allows you to place it anywhere on the tank.


  • Needs at least 30 psi to work and is not suitable for most DIY and low pressure CO2 systems
  • The ceramic plate is not removable and it is difficult to clean.
  • Often clogs and builds up mineralization that reduces flow

2. JARDLI pollen glass CO2 diffuser with bubble counter

  • Type of diffuser: Ceramic
  • Tank size: 20 gallons and under
  • Dimensions: 3.1 x 0.8 inches

Another option for pressurized CO2 systems would be this small glass diffuser from Jardli. The thicker ceramic membrane produces a fine mist of bubbles the size of pollen grains, which help the CO2 dissolve in the aquarium water. This is a simple but very attractive diffuser that sits inside your tank.

It comes complete with a built-in bubble counter, so if you don’t already have a bubble counter on your system, this might be a good choice. The ceramic plate is approximately 0.8 inches in diameter, so this diffuser would be ideal for nano tanks under 20 gallons in capacity.

The main disadvantages of this diffuser are related to the construction materials. Glass is fragile and you will need to remove the diffuser from your tank to clean it. You also can’t replace the ceramic disk, so if you break the bubble counter or glass tube, you’ll have to replace the entire device.

I also wouldn’t recommend this diffuser if you don’t have a pressurized CO2 system, because DIY and low pressure systems rarely produce enough psi to move the CO2 through the ceramic membrane. However, this is a good option for nano tanks with water space.


  • Attractive glass diffuser with built-in bubble counter that looks great in an aquarium
  • The ceramic membrane produces a fine mist of bubbles the size of pollen grains.
  • Ideal size for nano tanks less than 20 gallons in capacity


  • The ceramic membrane is not removable, so you will have to pull the entire diffuser out to clean
  • Ceramic membrane is thick and requires a pressurized CO2 system to make bubbles
  • Glass construction is brittle and breaks easily during maintenance.

3. Sera Flore CO2 Active Reactor 500

  • Diffuser type: Reactor
  • Tank size: 66 to 159 gallons
  • Dimensions: N/A

If you want the best CO2 reactor, this model from Sera is considered top of the line for smaller aquariums. It is the ideal size for a planted tank under 160 gallons and works with pressurized and low pressure CO2 injectors. With two impeller blades, the Sera can diffuse up to 500 CO2 bubbles per minute in your aquarium.

The Sera can be installed in-line as part of your canister filtration system, or you can use it with a pump inside your aquarium. This flexibility allows you to place it in the optimal location based on your tank’s flow patterns and the location of your inlet and outlet pipes.

CO2 reactors are substantially more efficient than other designs, so if you want to ensure that most of the CO2 in your system is available to your aquatic plants, using a reactor is the way to go. This is an ideal reactor for tanks starting at about 50 gallons, but would probably be a waste if installed in smaller planted tanks.

Like all CO2 reactors, the Sera is particularly noisy right after installation or maintenance, as the air bubbles need time to escape. This isn’t the easiest diffuser to install either, but Sera has top notch customer service and they are very helpful if you run into issues. I would definitely choose this diffuser over any other on my list if it is appropriate for your tank.


  • The reactor diffuses up to 500 CO2 bubbles per minute into your aquarium water
  • Can be connected to your canister filter as an in-line diffuser or fed with a separate pump
  • Can be mounted internally or externally to your tank


  • Noisy at first until the air bubbles have left the reactor.
  • Can be tricky to install if you’ve never used a reactor before
  • Construction is not as durable as it could be and acrylic mounts break easily

4.STARSIDE Aquarium CO2 Regulator Kit

  • Type of diffuser: Ceramic
  • Tank size: up to 100 gallons
  • Dimensions: 3.2 x 3.2 x 3.2 inches

If you like the look of a glass diffuser but want a more inclusive kit that’s a little less brittle than standard glass, this Starside CO2 Regulator Kit might be a good choice for your rimless tank. Resin-based glass is stronger and more flexible than normal glass and is less likely to break during maintenance.

This is a very attractive diffuser set as it would look great in most planted aquariums. The kit includes the ceramic diffuser, a check valve, a U-shaped glass tube, and a suction cup to secure the device in place inside your tank. The U-bend replaces the plastic tubing that bends so easily and reduces CO2 flow.

The large ceramic membrane makes this system ideal for tanks up to 100 gallons in capacity. However, it still requires a pressurized CO2 system and will not work well with low pressure generators. Resin-based glass is stronger and more flexible, but can still easily break during maintenance if you’re not careful.

The biggest problem with Starside is that the U-bend is too narrow to fit tanks with the normal bevel or rim around the top. You’d have to shave off the plastic to make room if you decide to try it on anything other than a rimless tank. But if you have a large rimless planted tank, this could be the perfect diffuser for you.


  • Attractive resin-based glass kit is less brittle than glass and ideal for small rimless tanks
  • Includes a check valve so you don’t have to buy one separately
  • Works with pressurized CO2 injection systems


  • U-shaped connecting tube is too narrow to fit on rimmed tanks
  • The pieces are more durable than standard glass, but are still delicate and break easily.
  • Will not work with most DIY or low pressure CO2 systems

5. JARDLI Glass Inline CO2 Atomizer Diffuser System

  • Diffuser type: inline
  • Tank Size: N/A
  • Dimensions: 5.3 inches tall

For an attractive and easy to clean inline CO2 diffuser, this beautiful design from Jardli might be the way to go. The large glass diffuser clips directly into your canister’s filtration system, so you don’t have to clutter your tank with an internal CO2 diffuser. Works with any canister filter that uses 0.5 inch wide tubing.

You can use this diffuser in any size tank, because its filter controls the water flowing through the ceramic membrane. You can adjust your CO2 levels simply by adjusting the flow rate of your filter and the amount of CO2 entering the line. It works best with pressurized CO2, but many have modified it to work with their DIY generators as long as the pressure isn’t too low.

However, this is not a complete kit. You’ll still have to use a check valve when you connect it to your CO2 canister, and a bubble counter makes estimating your CO2 levels much easier. Since this unit sits behind or under your tank, it is less likely to develop algae growth and is easier to remove for cleaning.

However, the glass is still fragile, so you will need to be careful during maintenance. This might not be the least expensive diffuser on my list, but next to Sera’s CO2 jets, it’s still one of the best options if you want to maximize your CO2 diffusion.


  • In-line installation reduces clutter inside your tank
  • Easier to maintain as it is easily accessible and less likely to grow algae if placed outside the tank.
  • Works with DIY and pressurized CO2 systems


  • It does not come with a check valve or bubble counter.
  • The ceramic disc is not replaceable, so the entire unit must be removed for cleaning and unclogging.
  • Glass is fragile, so you might break it when disassembling it for cleaning.


As you can see, there is a lot to consider when you are researching the best CO2 diffusers for aquariums. The ideal type of diffuser depends in part on the size of your aquarium, but is mostly related to the type of CO2 generator and filtration system you are running in your planted tank. We would love to hear about your experience with CO2 diffusers!

To sum things up quickly:

  • I would go with the Sera CO2 reactor for a 50+ gallon tank.
  • The Jardli Pollen Diffuser is an excellent option for nano tanks 20 gallons or less.
  • The Starside Kit is an ideal option for rimless aquariums and also includes a glass U-bend.

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