A CO2 reactor for an aquarium is a device that dissolves carbon dioxide gas into tank water with near-100% efficiency, making it the most effective way to deliver CO2 to a planted tank. Unlike diffusers that release bubbles into the water column with some CO2 escaping to the atmosphere, a reactor traps the gas in a sealed chamber and keeps it in contact with flowing water until it's fully dissolved. If you're running a high-tech planted tank and frustrated with CO2 waste or inconsistent plant growth, a reactor is the upgrade that makes the biggest difference.

CO2 injection is what separates a thriving planted tank from a struggling one. Plants use CO2 as a primary building block through photosynthesis, and in a sealed aquarium, CO2 is typically the limiting factor on growth rate. A reactor makes sure that every bubble you're paying for actually makes it into your plants rather than gassing off at the surface. This guide covers how reactors work, the different types available, how to install one, and how to dial in the right CO2 rate.

How a CO2 Reactor Works

The principle is straightforward. Tank water is pumped into the top of the reactor chamber, CO2 gas is injected into the same chamber, and the two are mixed together under pressure as water flows through. The chamber is sized so that the water's residence time is long enough for all the CO2 to dissolve before the water exits back into the tank. A good reactor produces zero visible bubbles in the outlet flow, which means 100% dissolution efficiency.

External vs. Internal Reactors

External reactors sit outside the tank or sump, connected by tubing to a powerhead or pump. They're the most efficient design and the easiest to maintain because you can access them without reaching into the tank. The Aquario Neo CO2 Reactor and the Green Leaf Aquariums Atomic CO2 Reactor are popular external models.

Internal reactors are submersed inside the sump or display tank. They're less common because they're harder to service and take up space inside the tank, but some nano tank builds use them where sump space isn't available.

How Dissolution Efficiency Compares to Diffusers

A standard ceramic diffuser releases bubbles into the water column. Larger bubbles dissolve partially on the way up and some CO2 escapes at the surface. Efficiency typically ranges from 50-80%, depending on tank depth and surface agitation. A reactor achieves 95-100% efficiency. On a system running 2 bubbles per second, this difference can mean 20-50% less CO2 consumption for the same dissolved CO2 concentration in the water.

Over a year of running a pressurized CO2 system, that efficiency difference adds up to meaningful savings on CO2 refills.

Types of CO2 Reactors

Inline Reactors

Inline reactors connect directly to your canister filter's outlet or inlet hose. Water flows through them as part of the normal filtration circuit without requiring a separate powerhead. This makes them the cleanest installation option. The Rhinox inline series and the Up Aqua CO2 Reactor connect in-line with standard 16/22mm or 12/16mm tubing.

The downside is that if you need to service the canister filter, you're also dealing with the reactor plumbing at the same time.

Hang-On Reactors

Hang-on reactors clip to the back of the tank and use a small powerhead to pull water through the chamber. These are common on tanks without canister filters where inline installation isn't an option. They're visible on the outside of the tank, which some people dislike, but they're functional and easy to clean.

DIY Reactors

Many planted tank hobbyists build their own reactors from PVC pipe or clear acrylic tubing. A basic DIY reactor can be built for under $20 in materials and performs comparably to commercial models. The principle is simple enough that there's no functional reason to spend $80-$150 on a commercial unit if you're comfortable with basic plumbing.

Setting Up a CO2 Reactor

Installation varies by reactor type, but the basic steps are consistent.

What You Need

  • CO2 cylinder (5 lb is the standard size for most hobbyists)
  • Regulator with solenoid and needle valve (Milwaukee MA957 or similar)
  • CO2-resistant tubing (standard silicone tubing degrades with CO2 over time; use polyurethane or specialized CO2 tubing)
  • Bubble counter
  • Drop checker for dissolved CO2 monitoring
  • The reactor itself, appropriate for your filter and tank configuration

For a complete CO2 setup including everything upstream of the reactor, our guide to the best CO2 system for aquarium covers regulators, cylinders, and components at every price point.

Connecting the Reactor

For an inline reactor, close the valves on your canister filter, disconnect the outlet hose at a convenient point, and insert the reactor inline. Most inline reactors have a specific inlet (where water enters from the canister) and outlet (where water returns to the tank or sump). Getting these reversed reduces efficiency significantly, so double-check the flow direction arrows before tightening connections.

Run the CO2 line from your regulator to the reactor's gas inlet, usually a barbed fitting on the side of the chamber. Use a hose clamp on all connections.

Dialing In the CO2 Rate

Start with a low bubble rate, around 1 bubble per second, and check your drop checker 24 hours later. A drop checker filled with 4 dKH reference solution and bromothymol blue indicator turns from blue (low CO2) through green (ideal, approximately 30 ppm) to yellow (too high). Adjust the needle valve on your regulator up or down by small increments and recheck each day until you're consistently green during the light period.

Fish stress is a useful indicator of CO2 that's too high. Gasping at the surface, rapid gill movement, and lethargy before the lights come on all suggest CO2 is too elevated. If you see these signs, reduce the bubble rate immediately and increase surface agitation temporarily to off-gas some CO2.

CO2 Reactor vs. Diffuser: Which Should You Use?

For tanks under 20 gallons, a quality diffuser like the Rhinox 2000 or the Aquario Neo Diffuser works well and is simpler to set up. Reactor efficiency gains matter less at small scale.

For tanks 30 gallons and larger running high light intensities, a reactor is worth the additional setup complexity. The efficiency gains translate to lower CO2 costs and more consistent dissolved CO2 levels because the reactor's sealed chamber acts as a buffer against pressure fluctuations from the regulator.

For a dedicated look at top reactor models, our best CO2 reactor roundup compares performance, flow compatibility, and value across the most popular options.

Maintaining Your CO2 Reactor

Preventing Algae Growth Inside the Chamber

If your reactor gets any light exposure, algae can grow inside the clear chamber. This reduces efficiency and eventually clogs the outlet. Wrap the reactor body with black tape or foam to block light. Some reactors come in opaque housings for this reason.

Clearing Air Locks

Sometimes air accumulates at the top of the reactor chamber instead of dissolving, creating an air lock that stops water flow. To clear it, briefly increase the CO2 rate to flush the bubble through, or disconnect the outlet briefly to let the trapped gas escape. This happens most often at startup or after moving the reactor.

Cleaning the Impeller

If your reactor uses an internal powerhead, the impeller needs periodic cleaning. Remove and rinse it with clean water every 2-3 months. A fouled impeller reduces flow rate through the reactor, dropping dissolution efficiency.

FAQ

How much CO2 should I inject through a reactor? Target 20-30 ppm dissolved CO2 in the water column during the light period. Use a drop checker with 4 dKH reference solution to verify. The required bubble rate varies depending on your reactor model, tank size, plant density, and surface agitation, so there's no universal number. Start at 1 bubble per second and adjust based on your drop checker readings.

Can I use a CO2 reactor with a canister filter that uses small tubing? Most inline reactors come in two hose size variants, 12/16mm and 16/22mm, to match common canister filter hose diameters. Check your filter's specifications before ordering. If your filter uses a size that doesn't match available reactors, a hang-on reactor with its own powerhead is the better solution.

Will a CO2 reactor harm my fish? A properly dialed CO2 system does not harm fish at the concentrations needed for plant growth. Problems occur when CO2 is set too high (above 40-50 ppm), CO2 is running during the dark period when plants aren't consuming it, or surface agitation is blocked so CO2 can't off-gas at night. Using a timer to turn off CO2 injection 1 hour before lights out prevents overnight CO2 buildup.

Do I need a CO2 reactor if I have a low-tech planted tank? Low-tech planted tanks use slow-growing plants that thrive under low light without CO2 supplementation. If you're running low light and keeping species like java fern, anubias, or crypts, no CO2 is needed at all. Reactors are for high-tech setups with demanding plants like carpeting species, stem plants requiring high light, or fast growers.

Bottom Line

A CO2 reactor is the most efficient way to get CO2 into a planted aquarium. If you're already running a pressurized CO2 system with a diffuser and want better results with lower CO2 consumption, swapping to a reactor is a direct upgrade. Install it inline if your canister filter allows it, aim for a green drop checker reading during the light period, and block light from reaching the reactor body to prevent algae inside the chamber.