An aquarium reactor is a sealed chamber that forces water into close contact with a reactant media, dissolving or exchanging substances into the water column far more efficiently than open-top methods. The most common types are calcium reactors (used in reef tanks to maintain calcium and alkalinity), CO2 reactors (for planted freshwater tanks), media reactors (for carbon, GFO, or biopellets), and phosphate reactors. Which type you need depends entirely on what kind of tank you're running.
If you keep a reef aquarium with stony corals, a calcium reactor is one of the most reliable long-term dosing methods available. If you have a planted freshwater tank, a CO2 reactor helps dissolve carbon dioxide efficiently. And if you're fighting phosphate or nitrate problems, a media reactor running GFO or biopellets can make a significant difference. This guide covers how each type works, what to expect when you set one up, and how to pick the right one for your situation.
How a Calcium Reactor Works
A calcium reactor is a pressurized chamber filled with calcium carbonate media, usually aragonite or crushed coral. CO2 is injected into the chamber, which drops the pH inside below 6.5. At that pH, the water dissolves the calcium carbonate media, releasing calcium and carbonate alkalinity (bicarbonate) into the effluent water that drips back into the tank.
This mimics the natural process by which coral skeletons are built, and it's one of the most cost-effective ways to maintain stable calcium and alkalinity in a heavily stocked reef. Instead of buying two-part dosing solutions every week or topping off a kalkwasser drip container, you refill media a few times a year.
Key Components
A calcium reactor system includes the reactor body, a CO2 cylinder, a CO2 regulator, a recirculation pump inside the reactor, a feed pump, a bubble counter, and a pH controller or probe. Brands like Reef Octopus, Korallin, and Two Little Fishies (TLF) make well-regarded reactors. The Two Little Fishies PF601S is a popular mid-size option for tanks up to about 200 gallons, while the Reef Octopus CR200 handles up to 100 gallons and is known for reliable flow control.
Dialing One In
Setting up a calcium reactor takes patience. You adjust CO2 bubble rate and effluent drip rate together until your tank's calcium and alkalinity stay stable between water changes. A pH controller like the Neptune Systems Apex or Milwaukee pH controller can automate CO2 injection to maintain the target pH inside the chamber. Expect to spend a week or two tuning before it runs hands-free.
CO2 Reactors for Planted Tanks
In a planted freshwater aquarium, CO2 injection dramatically improves plant growth. But CO2 dissolves poorly at the surface, so getting efficient dissolution matters. A CO2 reactor does this by trapping CO2 bubbles inside a chamber with circulating water until they dissolve completely before the water continues into the tank.
Without a reactor, CO2 is often added via a diffuser, which releases small bubbles that partially escape to the surface before dissolving. A reactor is more efficient, achieving close to 100% dissolution. This matters because CO2 injection costs money (either from disposable cartridges or a pressurized CO2 system), and wasting it to surface off-gassing is frustrating.
Popular CO2 reactor options include the Fluval Ceramic CO2 Diffuser Kit (affordable entry-level), the Aquario Neo CO2 Diffuser Reactor (strong reviews for mid-size tanks), and the Green Leaf Aquariums inline reactor (efficient design that connects directly to your canister filter line). The inline style is particularly clean since it hides inside the filter tubing run rather than sitting visibly inside the tank.
Media Reactors: Carbon, GFO, and Biopellets
A media reactor is a powered chamber that tumbles or fluidizes a media bed. Three common applications:
Carbon Reactors
Running activated carbon in a reactor is cleaner and more efficient than stuffing it in a mesh bag inside a filter. The fluidized bed exposes all surfaces of the carbon granules to water flow, improving adsorption of dissolved organics, yellowing compounds, and medications. Brands like BRS (Bulk Reef Supply) make affordable single-chamber reactors that work well for carbon and GFO.
GFO (Granular Ferric Oxide) Reactors
GFO binds phosphate from the water column. It's a popular option for reef keepers dealing with phosphate-driven algae problems. Running GFO in a reactor rather than a bag is important because GFO needs to tumble slowly without clumping. Too much flow causes the beads to break down and release phosphate back into the water. The Aquarium Plants Factory dual-chamber reactor and BRS Single Reactor are frequently recommended for this use.
Biopellet Reactors
Biopellets are a solid carbon source that feeds nitrate-reducing bacteria. They need to tumble vigorously in a dedicated reactor. As bacteria colonize the pellets, they consume nitrates and phosphates, then get exported via the protein skimmer. BRS and TLF both make biopellet reactors. This approach works well in heavily stocked reef systems but takes 6-8 weeks to fully establish.
Phosphate Reactors
Sometimes called a phosban reactor, these are essentially the same as media reactors but specifically designed and sold for phosphate control. The Two Little Fishies Phosban Reactor 150 is probably the most commonly seen version, a compact design that's been around for years. It runs GFO, carbon, or biopellets. For a 75-150 gallon reef, the 150 size handles the flow rate well.
Choosing the Right Reactor for Your Tank
A few questions narrow down your decision quickly.
What type of tank? For planted freshwater, you need a CO2 reactor. For reef tanks, calcium reactor or media reactors (or both). For FOWLR (fish only with live rock), media reactors for water clarity may be useful but calcium reactors aren't needed.
How large is your tank? Reactors are sized by flow rate and media volume. A calcium reactor sized for 50 gallons won't keep up with a 200-gallon SPS-dominated reef. Check manufacturer recommendations and go slightly larger rather than undersizing.
What's your budget? Calcium reactors with a CO2 setup run $200-600 for the reactor plus $50-150 for a regulator and CO2 cylinder. Media reactors are much cheaper, typically $30-100.
For a broader look at what equipment makes sense at each stage of a reef or planted build, browse our best aquarium equipment guide.
Installation Tips
Inline installation (between the canister filter outlet and return) keeps equipment out of the sump or display tank. Most CO2 and media reactors support this with standard 1/2" or 5/8" tubing connections.
For calcium reactors, running the effluent drip into an area with high flow in the sump helps prevent pH spikes near corals. A secondary chamber (like the Korallin C3002) can raise the effluent pH back up before it enters the tank, which reduces the impact on main tank pH.
Check your flow rate before purchasing. Most media reactors list a GPH (gallons per hour) flow rate range on the label. Exceeding this flushes media out before it can work; running too low risks dead spots and channeling.
For more options and comparisons across the full range of aquarium equipment, our top aquarium equipment roundup covers reactors alongside filters, lights, and skimmers.
FAQ
Do I need a reactor for a freshwater aquarium? Not necessarily. A CO2 reactor is very useful if you inject CO2 in a planted tank, but a simple diffuser works adequately in smaller setups. A media reactor for carbon or chemical filtration is optional for freshwater tanks and rarely essential.
How often do I need to refill a calcium reactor? This depends heavily on your tank's calcium demand and the media density. A moderately stocked reef of 100 gallons typically needs a media refill every 2-4 months. High-demand SPS tanks go through media faster.
Can I run a CO2 reactor and a calcium reactor on the same CO2 cylinder? Technically yes, but it complicates flow and pressure management. Most reef keepers run separate regulators and cylinders for each application to keep adjustments independent.
What happens if my calcium reactor's CO2 runs out? The reactor stops acidifying the media, so calcium and alkalinity output drops to near zero. Your tank's parameters will drift down over time. Monitoring your calcium and alkalinity weekly lets you catch this before it causes coral stress.