CO2 in Your Planted Aquarium: a Complete Guide to Reactors & Diffusers

Carbon dioxide fertilization is one of the most important upgrades you can do to have success with live aquatic plants.

My picks are the Flux and the Flore, keep on reading for more options!

Elemental carbon is an essential nutrient required by plants. Aquatic plants adsorb carbon from the water. When the carbon supply is depleted, aquatic plants stop growing. They may drop their leaves, turn yellow or melt away.

Considering that planted aquariums usually have bright lighting and fertilizer that it would be easy for algae to take over the tank. When aquatic plants are thriving they out-compete the algae for nutrients.

There is also evidence that the plants release natural anti-algae substances into the water. However, when a key nutrient is lacking, plant growth slows down or stops. This usually results in algae taking over the aquarium.

There are many liquid and tablet aquatic fertilizers for aquarium plants, but none add CO2. The problem is aquariums contain very little natural carbon dioxide compared to tropical lakes and streams. What traces of carbon dioxide are present are quickly removed by the plants. Excess aeration can also drive CO2 gas out of the water.

The most effective way to correct a carbon deficiency is by adding carbon dioxide gas to the aquarium.

How to add carbon dioxide to your aquarium

The first thing you’ll need is a supply of carbon dioxide gas.

The two most-used sources of CO2 are pressurized bottles of gas and yeast reactors.

Pressurized canisters and tanks of CO2 are available from aquarium suppliers and local sources. Some smaller canisters are disposable. Larger steel or aluminum tanks can be refilled at welding shops and beverage distributors.

Yeast reactors ferment sugar, producing small amounts of carbon dioxide gas.

Most aquarists choose pressurized CO2 gas due to simplicity and reliability. All CO2 is the same as far as the plants are concerned. You cannot simply “bubble” CO2 into the aquarium with an airline. The big gas bubbles will quickly rise to the surface and evaporate into the air. The CO2 must diffuse into the water so it can be adsorbed by the plants.

Diffusers and reactors increase the contact time with CO2 gas and water. The flow rate of gas entering the aquarium is usually visually metered with a bubble counter.

The bubble counter is a small water-filled device that is placed outside the aquarium. The rate of CO2 injection into the aquarium is determined by counting the bubbles per minute.

Aquarists determine how many bubbles per minute are required for optimal fertilization and plant growth. We’ll look at the most effective devices for increasing the level of carbon dioxide in the planted aquarium.

Pressurized diffusers

Diffusers are designed to force CO2 gas through a porous diffuser plate. The tiny pores in the diffuser create very small gas micro-bubbles that look like mist or pollen.

The diffuser is submerged into the aquarium and attached to the glass with a suction cup. As the micro-bubbles rise they dissolve into the water.

It takes relatively high pressure to force the gas through the diffuser plate. Yeast reactors do not create enough pressure and won’t work with “pollen-type” diffusers.

Pressurized CO2 gas is required for these diffusers. Mounting a diffuser is easy. Just press the suction cups against the aquarium glass. Diffusers should be mounted near the bottom of the aquarium. The farther the bubbles rise the more gas dissolves into the aquarium water.

Diffusers are usually most efficient in aquariums up to 30 gallons.

CO2art Inline Atomizer & In-tank Diffuser

The CO2 Art In-Tank Flux diffuser uses a porous ceramic membrane to create a fine stream of CO2 bubbles inside the aquarium.

The Flux diffuser mounts inside the aquarium. It is designed for use with pressurized CO2 systems.

The diffuser is positioned horizontally, creating a dense mist of rising CO2 bubbles. Flux diffusers are sized for tanks up to 65 gallons. A larger model is available for 65 gallon and larger tanks.

The CO2 Art In-line CO2 diffuser is connected to the return hose of your canister filter.

Pressurized CO2 enters the diffuser and passes through a ceramic membrane, creating a fine mist of carbon dioxide bubbles. As water flows from the canister, the In-Line diffuser injects CO2 into the water stream.

The In-Line diffuser eliminates the need for a CO2 diffuser inside your planted aquarium.


  • High quality construction.
  • Easy to use.
  • Ceramic membranes can be cleaned or replaced.


  • Requires high-pressure gas system.

JARDLI Pollen Glass CO2 Diffuser

The JARDLI pollen glass diffuser comes in two sizes; 1.5” for tanks up to 30 gallons and 2” for tanks between 50 – 75 gallons.

The porous ceramic membrane breaks up the gas into tiny micro-bubbles.

The diffuser is intended for pressurize gas sources that can create a flow rate of 60 bubbles per minute.

The JARDLI diffuser mounts inside the aquarium with suction cups.


  • Inexpensive.
  • Makes lots of tiny micro-bubbles.
  • Proven CO2 injection method.


  • Could become clogged with algae.

Rhinox Spio III CO2 Diffuser

The Spio III incorporates a spiral bubble counter inside the glass diffuser body.

This makes it easier to see the flow of gas as it travels through the spiral glass tube and through the ceramic diffuser.

The hand-made diffuser is rated for 20 + gallon aquariums.

A nano version is available for smaller aquariums, but it does not include the spiral bubble counter.

The Rhinox diffusers mount to the glass with a suction cup.


  • Spiral bubble counter.
  • Two sizes available.
  • Cool design.


  • Ceramic disc will require cleaning.

Inline CO2 Atomizer Diffuser

An inline atomizer is simply a diffuser placed inside a larger tube.

This allows a diffuser to be positioned outside of the aquarium, connected directly to a canister filter.

The idea is to combine water flow with diffusion.


  • Removes diffuser equipment from the aquarium.


  • Lacks the diffusion properties of a traditional in-tank diffuser.
  • Can’t be used on larger aquariums like reactors.
  • Places a delicate glass tube where it could be accidental cracked when servicing the canister filter.
  • Most smaller aquariums don’t use canister filters.

Carbon Dioxide Reactors

Unlike CO2 diffusers, carbon dioxide reactors don’t release CO2 gas directly into the aquarium. Instead, aquarium water and CO2 are mixed together in a separate container (reactor).

The idea is to mix the gas with the aquarium water then release the CO2-rich water back into the tank. In the past, reactors were quite large, over 12 inches tall and were placed outside of the aquarium. The large reactor was intended to maximize the mixing of water and CO2. Today CO2 reactors are much smaller.

A reactor can be positioned inside the aquarium or operated in-line with a canister filter. Carbon dioxide reactors are typically used on aquariums 30 gallons and larger.

Gulfstream Tropical Ista Max Mix CO2 Reactor

The Gulfstream reactor can be used externally or mounted inside the aquarium.

It’s designed to be connected to the outflow of a canister filter.

Water fills the reactor and spins the impellers inside the chamber. Carbon dioxide gas enters at the top of the reactor.

The spinning impellers increase contact time with the gas. The CO2-rich water is then pumped into the aquarium.


  • Inexpensive.
  • Internal or external mounting.


  • Suction cups may not hold reactor when mounter externally.
  • Some reactors may leak.
  • Hose clamps are not sturdy.

Sera Flore Active CO2 Reactor 500

The Flore Active Reactor 500 is a well-built, heavy-duty unit.

Water from a separate pump or canister filter enters the reactor through a secure fitting.

Carbon dioxide is fed into a hose barb at the top of the reactor.

The water and gas are mixed by spinning paddle wheels inside the chamber.

The water and bubbles mix and are forced to the bottom of the reactor. From there the water flows up a tube and back out to the aquarium.

The 500 model is rated for aquariums from 66 to 160 gallons. The Flore Active reactor can be opened up for cleaning.


  • Heavy-duty acrylic construction.
  • Quiet operation of the paddle wheels.
  • Leak-free operation.
  • Easy installation and disassembly for cleaning.
  • Internal or external mounting.


  • None.

Aqua Medic CO2 Reactor 1000

The Aqua Medic CO2 Reactor is a sealed reactor designed for internal or external mounting.

External mounting recommended due to the size of the unit.

Aquarium water enters the at the top of the reactor and trickles over the plastic balls.

CO2 gas flows upward in a counter-current pattern to increase contact time with the water.

The plastic balls create turbulence and increase absorption of CO2 into the water. Water flow should be a minimum of 250 gallons per hour. Aqua Medic CO2 Reactor 1000 measures 3" in diameter and 15" long.

The reactor is rated for aquarium up to 500 gallons.


  • Solid construction.
  • Comes with needle valve for CO2 adjustment.
  • Can be mounted under the aquarium cabinet.
  • High-quality hose fittings.


  • May require a separate water pump.
  • Larger reactor takes up space.

My Pick

For smaller aquariums either the Flux or the JARDLI will work well to inject CO2 into the water.

For larger aquariums, the Flore Active or Aqua Medic are recommended due to their high-quality construction and leak-free performance.

The Aqua medic reactor uses no moving parts that could jam or get dirty. It never needs to be opened up for cleaning. The set it and forget it operation of the Aqua Medic makes it our favorite CO2 reactor.