CO2 Reactor vs. CO2 Diffuser – What is the Difference?

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CO2 is crucial in a planted aquarium. This nutrient helps plants photosynthesize and grow. The more plants you have, the more CO2 you’ll need to keep up with the demand.

The best way to add CO2 to your tank is via either a CO2 diffuser or a reactor.

Either of these two pieces of equipment can be lifesaving for newbie aquarists. However, there’s an important distinction to make. CO2 diffusers and reactors are each suited for specific aquarium setups.

You should first know when to choose one over the other. So, what’s the difference, and which one is right for you? Well, let’s take a closer look at both!

What is a CO2 Diffuser?

A CO2 diffuser is an essential component in any CO2 system, alongside a CO2 cylinder and a CO2 regulator. The role of the CO2 diffuser, unsurprisingly, is to diffuse CO2 into the water. Here’s how it works—

The diffuser uses tubing to connect to the CO2 regulator. The regulator releases CO2-rich bubbles, which the diffuser then takes up. The bubbles within the diffuser travel through a porous material and break down into a mist.

Finally, because the bubbles are much smaller, the CO2 is readily absorbed by the water.

There are various types of diffusers for all price ranges and aquarium sizes. Some of the most common models you’ll find are:

– Airstone diffusers

These types are the cheapest and most subtle but also the least effective. Not to be confused with regular airstones, CO2 airstones contain special ceramic discs to facilitate CO2 diffusion.

These discs should create finer bubbles than a standard airstone. Airstone diffusers are also directly connected to a CO2 regulator.

Still, this type of diffuser is hardly efficient and not a top recommendation for most aquariums. The bubbles it releases are still too big for proper CO2 diffusion. Thus, not a lot of gasses get dissolved into the water.

Airstone diffusers create an aesthetically-pleasing aquarium, but with very low functionality. If you have anything other than a nano-tank, look elsewhere for a source of CO2.

– Ladder diffusers

A ladder diffuser is a suitable step-up from an airstone. As the name suggests, this diffuser model is shaped like a ladder. It rests neatly against the aquarium wall, blending seamlessly with the decorations.

This diffuser is also fully submerged and compact.

This diffuser is suitable for smaller aquariums. It dissolves an appropriate quantity of CO2, even for heavily planted tanks.

This practical and nice-looking diffuser also comes at an affordable price! For aquariums between 10-20 gallons, this is a top choice!

– Ceramic and glass diffusers

This diffuser and the airstone diffuser are based on a similar concept. Both models use a porous ceramic material to create smaller CO2 bubbles. But ceramic and glass diffusers have a higher output and disperse CO2 more effectively.

This diffuser is typically made up of a glass cylinder with an inner spiral tube and a ceramic disk as a cap.

The ceramic disk has very fine pores that create smaller bubbles than an airstone.

Such diffusers are highly effective and best suited for medium-sized tanks. They’re also nicely priced, considering their performance and utility.

This model is a good investment for a planted tank between 15-60 gallons.

– Inline diffusers

The last of the more popular diffuser models is the inline atomizer. This type of diffuser does it all— it’s highly effective for CO2 injection, is consumption-efficient, and is not submerged in the aquarium.

This is the product if you’re looking for a diffuser that won’t visually “pollute” your aquarium!

Inline diffusers are connected directly to the canister filter and can be tucked away with the rest of the CO2 system components. They’re easy to find and come in virtually any diameter, so tubing won’t be an issue.

An inline diffuser can manage a high CO2 input and is best suited for large aquariums. This is the most cost-effective and efficient option if your tank is up to 100 gallons or over.

This diffuser model can handle any aquarium size up to 500 gallons.

Of course, these were just the most popular models. There are also more expensive and specialized diffusers, best suited for very large aquariums.

But the ones I’ve described so far will cover 99% of all hobby aquarists reading this.

What is a CO2 Reactor?

People sometimes mix up diffusers and reactors. They aren’t the same thing, but they’re pretty close. A CO2 reactor is a particular type of diffuser.

Its role is also to diffuse CO2 bubbles. But it gets a different name, so things get confusing. Simply put, a CO2 reactor is an injection method that achieves complete or near 100% diffusion.

In contrast, regular CO2 diffusers only achieve partial CO2 dissolution.

A reactor is the most effective CO2 injection method, but also expensive and tricky to install. It differs from other diffusers because it can operate both inside and outside the aquarium.

Unlike other diffusers, a CO2 reactor is also discreet. It increases the CO2 concentration in the water without creating bubbles or mist. You won’t even know the reactor is there. How does it do that?

Well, a CO2 reactor looks similar to an inline diffuser. But it circulates the CO2 differently. Once the CO2 enters the reactor, it won’t travel out immediately. It remains and keeps circulating inside the chamber until it is fully dissolved.

Thus, you get no bubbles! Also, because the CO2 is completely dissolved, you get a very high output and near-complete dissolution into the water.

CO2 reactors have one significant disadvantage, or advantage, depending on how you look at it. They require a high gallon-per-hour rate to work correctly.

This means they’re best suited for very large aquariums. But certain products can also work on medium-sized tanks.

As you can expect, the many perks and high output come at a price. Literally. You can expect to pay upwards of $50 for a small-output reactor if you have a medium-sized tank. The high-end products geared towards large aquariums go over $100 easily.

Pros & Cons of CO2 Reactors

Is the CO2 reactor the right choice for you? Well, here’s a list of the pros & cons to help you decide!


  • Near total CO2 diffusion
  • High efficiency means less CO2 consumption
  • No bubbles or mist
  • Can work internally and externally
  • Needs less pressure to operate
  • Long-lasting


  • Challenging to install
  • Most models need special tubing
  • Can be very expensive
  • Mostly suited for large aquariums
  • Needs high GPH output to operate optimally
  • It may be noisy, depending on the reactor content and output

Reactors provide the best CO2 diffusion, but you may not need this if you have a medium-sized aquarium.

Unless you dislike seeing water bubbles in your tank, investing in a reactor can be overkill.

Pros & Cons of CO2 Diffusers

If you have a small or medium-sized aquarium, you’re probably already considering a regular CO2 diffuser. Let’s see how it compares to a reactor.


  • Affordable
  • Many models to choose from
  • Can work internally or externally, depending on the type
  • Easy to install
  • Efficient CO2 diffusion, depending on the type
  • Suitable for most tank sizes, including nano


  • Most types work internally
  • Can’t achieve 100% CO2 dissolution
  • Diffusers create bubbles and mist
  • Most types require higher pressure from the regulator
  • Submerged diffusers aren’t the most durable

The quality and efficiency of diffusers vary a lot. Your results will depend on the type of diffuser and the brand you choose.

Always remember to double-check the suitable tank capacity when buying a product.

Are Inline CO2 Diffusers Good?

Inline CO2 diffusers have a good reputation. They’re pricier, but also safe and very effective. An inline diffuser is a perfect compromise between a regular diffuser and a high-end CO2 reactor.

It has most of the benefits of a reactor but is more accessible and can also work for medium tanks.

Inline diffusers operate externally, so they won’t take up space in the tank. They produce very fine bubbles, achieving higher CO2 diffusion.

The high output makes them more energy-efficient than most other diffuser models.

Inline diffusers even look quite similar to reactors. The sturdy design makes inlines more durable than the regular internal diffusers.

Finally, inlines offer more leeway because they work on most regular aquarium sizes. You can use this diffuser type on medium, large, and very large tanks.


CO2 diffusers and reactors achieve the same end goal— dissolving CO2 into the aquarium. However, they do so through various methods. There are still considerable differences between these two pieces of equipment.

Distinguishing between reactors and diffusers comes in handy when creating the best setup for your tank.

Diffusers are affordable and come in a variety of models. They work by breaking down CO2 into fine bubbles. They achieve suitable CO2 dissolution for aquariums of various sizes.

You can find diffusers geared towards small, medium, and even large tanks. Of all diffuser models, inlines are the most efficient.

CO2 reactors are the best of the best. They achieve total CO2 dissolution and create no bubbles or mist.

However, they’re only suited for large and very large aquariums. They’re also costly and not as easy to operate. For general purposes, inline diffusers provide a more affordable and user-friendly alternative.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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