CO2 Reactor — External

External CO2 reactors are widely regarded as the most efficient method of CO2 dissolution available to planted aquarium hobbyists. This reactor installs inline on the outlet of a canister filter and uses a spiral mixing chamber to trap CO2 bubbles and force them into prolonged contact with the flowing water until they dissolve entirely. Unlike diffusers that release micro-bubbles into the water column, the reactor retains CO2 inside the chamber until fully absorbed, achieving near 100% dissolution efficiency with no gas reaching the water surface. This makes external reactors the preferred choice for large or high-demand planted tanks where maximum CO2 efficiency is essential. A small amount of residual gas is periodically purged via the automatic or manual bleed valve.