Inline CO2 Reactor: A Guide for Large Planted Tanks
An inline CO2 reactor is a chamber plumbed into a canister filter line that dissolves injected carbon dioxide almost completely before water returns to the tank.
What it is
An inline CO2 reactor is a pressurised chamber installed on the outflow line of a canister filter. It traps injected carbon dioxide and mixes it with the passing water until the gas dissolves, so the return water carries dissolved CO2 into the tank rather than visible bubbles. It is an alternative to in-tank diffusers.
How it works
Gas is fed into the reactor body, where filter flow swirls and mixes it until it is fully absorbed. Because the gas is held in the chamber and forced to dissolve before exiting, dissolution is far more complete than with a diffuser, which lets some bubbles escape the surface before they dissolve. Reactors require no separate power, running on the existing filter flow.
Reactor versus diffuser
An in-tank ceramic diffuser breaks CO2 into a fine mist that the water column slowly absorbs, but a portion of that gas reaches the surface and is lost before dissolving, and the mist is visible in the display. A reactor instead traps the gas and dissolves it inline with minimal loss, so no mist clouds the tank. The diffuser is simpler and cheaper for small tanks, while the reactor is favoured where high efficiency and a clean look matter on larger volumes.
Why use one
- Very high dissolution efficiency with minimal gas loss
- No visible mist or bubbles clouding the display
- Equipment stays hidden in the cabinet, inline with the filter
- Well suited to large or densely planted aquariums needing consistent CO2
Drawbacks
A reactor adds resistance to the filter line, which can reduce water flow, and excessive gas buildup inside the chamber can further restrict flow until it is purged. Setup is more complex and takes more space than a simple diffuser, which is why reactors are generally chosen for advanced, larger systems.
Installation
The reactor is plumbed inline on the canister filter's return line, with the CO2 feed connected to its inlet. Tubing, valves, and seals must be matched to the filter and checked for leaks. Because it relies on filter flow, the filter must have enough capacity to overcome the added resistance.
Maintenance
Periodic inspection keeps a reactor working: tubing, valves, and seals should be checked, and accumulated gas purged if flow drops. Internal surfaces may collect debris or algae over time and need cleaning, though usually less frequently than a ceramic diffuser disc.