An external CO2 reactor dissolves carbon dioxide gas into aquarium water with near-100% efficiency by running tank water and CO2 through a sealed chamber where the gas is diffused before the water returns to the tank. This is the most efficient way to deliver CO2 to a planted aquarium, significantly outperforming inline diffusers and airstone-style diffusers that release bubbles into the water column where much of the CO2 escapes before dissolving.
If you're running a pressurized CO2 system on a planted tank and your diffuser isn't keeping up with plant demand, or you're seeing CO2 bubbles reaching the surface undissolved, an external reactor is the upgrade worth making. I'll explain how these devices work, what makes one better than another, how to install one, and what you can expect to pay.
How an External CO2 Reactor Works
An external reactor is a sealed cylinder typically installed inline on the outlet hose of your canister filter. Water enters from the top (or bottom, depending on design), CO2 is injected into the chamber through a small inlet, and the combination is tumbled or mixed inside the chamber until all the CO2 dissolves into the water before it exits back to the tank.
The key advantage over a standard glass diffuser is that the water stays in contact with the CO2 for much longer. A diffuser releases CO2 as small bubbles into the open water column. Some bubbles dissolve, but many reach the surface and escape. A reactor keeps the gas sealed with the water until dissolution is complete.
Better reactors use an impeller or a spiral baffle inside the chamber to increase surface contact between the water and gas. The longer the contact path and the more turbulent the mixing, the more efficiently the CO2 dissolves. In a well-designed reactor, you should see zero CO2 bubbles exiting the outlet, which means you're getting full dissolution.
Types of External CO2 Reactors
Inline Reactors
Inline reactors sit on the outlet hose between your canister filter and the tank return. They're invisible inside the tank, which many aquascapers prefer for a clean look. Installation requires cutting your return hose and inserting the reactor in line.
The Aquario Neo Reactor M ($30 to $45) is one of the most popular inline reactors available. It uses a compact spiral design that fits inline on hoses from 9mm to 16mm ID. At this price point, it's hard to beat for tanks under 75 gallons.
The UP Inline CO2 Atomizer ($15 to $25) is a budget option that works on smaller hoses (12mm to 16mm). It's less efficient than the Neo Reactor but gets the job done for tanks under 40 gallons on a tighter budget.
For larger planted tanks (75 gallons and above), the Aquario Neo Reactor L ($45 to $65) or the GLA (Green Leaf Aquariums) External CO2 Reactor ($55 to $80) handle higher flow rates without restricting the canister filter's output.
Hang-on-Back Reactors
Hang-on-back reactors mount on the tank rim and use a small pump to circulate water through the chamber. They don't require cutting your filter hose and are easier to install. They're a better option if you're running a hang-on-back filter without a canister.
The Rhinox 1000 ($20 to $35) is a widely used hang-on-back reactor for tanks up to 80 gallons. It uses a simple chamber design and is easy to clean and maintain. CO2 efficiency with hang-on-back reactors is slightly lower than inline units because water flow through the chamber is driven by a secondary pump rather than the full pressure of your canister filter, but the difference is minor in practice.
Sizing a Reactor for Your Tank
The most important sizing consideration is flow rate. The reactor must handle the full output of your canister filter without creating backpressure that noticeably reduces flow. Most reactors specify a maximum flow rate in GPH or liters per hour.
Match the reactor size to your filter's rated output with a 20 to 30% margin:
- Tanks under 40 gallons: Aquario Neo Reactor M (rated to about 300 LPH / 80 GPH)
- Tanks 40 to 75 gallons: Aquario Neo Reactor L or GLA External Reactor (rated to 400 to 600 LPH)
- Tanks over 75 gallons: GLA External Reactor XL or custom reactor designs
CO2 Injection Rate
The reactor itself doesn't determine how much CO2 you inject. Your CO2 regulator controls bubble rate. The reactor just ensures that whatever CO2 you're injecting dissolves fully rather than escaping as bubbles.
For planted tanks, target CO2 levels of 20 to 30 ppm in the water. Start at 1 bubble per second per 10 gallons and adjust based on plant response and CO2 drop checker readings.
Installation: Step by Step
Installing an inline reactor is straightforward if you take your time.
- Turn off your canister filter and pinch or clamp the return hose to prevent draining.
- Select the installation point. Most aquascapers install the reactor on the return hose, as close to the tank as practical, so filtered water passes through the reactor before entering the tank.
- Cut the return hose and connect the reactor. Use the barb fittings and clamps included with the reactor. Ensure the cut is clean and straight so the fittings seat properly.
- Connect your CO2 supply line to the reactor's CO2 inlet. Use the check valve from your CO2 line here to prevent tank water from back-siphoning into the regulator.
- Restart the canister filter and check all connections for leaks before tightening clamps fully.
- Adjust CO2 bubble rate at the regulator. Within 30 minutes you should see no bubbles exiting the reactor outlet, confirming full dissolution.
If small bubbles do exit the reactor, either the CO2 rate is too high for the reactor volume or your canister flow rate is too high for the reactor size. Reduce the CO2 injection rate first and see if the bubbles clear.
Maintenance and Common Issues
External reactors need minimal maintenance but a few issues come up regularly.
Bubble Accumulation
CO2 gas can accumulate at the top of the reactor chamber over time, particularly in horizontal-mount designs. This reduces efficiency. Most reactors have a purge screw or can be tilted to release accumulated gas. Some hobbyists add a small check valve on the CO2 inlet to prevent excess buildup.
Algae Growth
If your reactor receives light, algae can grow inside the chamber and on the impeller. Monthly disassembly and cleaning with a bottle brush prevents this. Store the reactor out of direct light if possible.
Flow Rate Changes
A clogged reactor impeller or debris buildup can reduce flow from your canister filter noticeably. If you notice your return flow decreasing, clean the reactor first before assuming the filter needs maintenance.
If you're planning a full CO2 system for a planted tank, our Best CO2 System for Aquarium guide covers regulators, cylinders, and diffusers alongside reactor options. For reactor-specific comparisons, see our Best CO2 Reactor roundup.
FAQ
Do I need an external reactor if I already have a diffuser?
Only if your diffuser isn't keeping up. Signs that a diffuser is underperforming: you see large bubbles reaching the surface undissolved, your CO2 levels are inconsistent despite steady injection, or your plants show CO2 deficiency symptoms (yellowing, slow growth, algae outbreaks) despite reasonable bubble rates. A reactor will solve all three of these by ensuring complete dissolution.
Can I use an external reactor with a hang-on-back filter?
You can use a hang-on-back style reactor (like the Rhinox 1000) without a canister filter. Inline reactors require a canister filter because they run on the filter's hose pressure. If you're running a hang-on-back filter for your main filtration, a hang-on-back reactor or a powerhead-driven reactor is the better choice.
Will an external reactor affect my filter flow rate?
A properly sized reactor has minimal impact on flow. An oversized or partially clogged reactor can create backpressure that reduces your filter's output by 10 to 20%. Monitor your return flow after installation and clean the reactor regularly.
How often do I need to replace or clean the reactor?
Disassemble and clean the reactor every 4 to 8 weeks, depending on how much algae and debris accumulates. The reactor itself has no consumable parts; unlike CO2 cartridges or regulator membranes, it doesn't wear out and can last for years with basic maintenance.
What to Buy
For most planted aquariums, the Aquario Neo Reactor M is the best starting point. It's reliable, efficient, easy to install, and priced fairly at $30 to $45. Move to the Neo Reactor L or the GLA External Reactor if you have a larger tank or a high-flow canister filter. Skip the cheapest no-name reactors; poor build quality and weak fittings cause leaks that create more problems than they solve. A good reactor lasts years and makes your CO2 system work at its full potential from day one.