A marine tank reactor is a sealed chamber device that dissolves or processes a media or chemical continuously in a controlled flow of water, then doses the processed water back into your tank or sump. The most common types are calcium reactors (which dissolve calcium-rich media to supplement calcium and alkalinity for coral), nitrate reactors (also called sulfur denitrators, which reduce nitrate biologically), phosphate reactors (which run granular ferric oxide or GFO media), and carbon reactors (which run activated carbon for chemical filtration). Each type solves a specific water chemistry problem, and most reef keepers eventually run at least one or two types simultaneously.

This guide covers the main reactor types used in marine tanks, what each does, how they're plumbed and configured, which tanks benefit from them, and what to look for when choosing one.

Calcium Reactors

A calcium reactor is the most significant investment in the marine reactor category and the most impactful for SPS reef tanks. It dissolves calcium carbonate media (typically aragonite or ARM media) in a slightly acidic chamber, releasing calcium, alkalinity (as bicarbonate), and trace minerals continuously into the sump.

How a Calcium Reactor Works

CO2 is injected into the sealed reactor chamber, dissolving into the water to form carbonic acid. This lowers chamber pH to around 6.5 to 6.8, at which point the calcium carbonate media dissolves. A recirculation pump keeps water and gas mixing inside the chamber. Processed water (the effluent) drips out through an adjustable valve into the sump at a rate you control.

The effluent has elevated calcium (usually 400 to 600 ppm in the chamber output), elevated alkalinity, and a low pH of around 6.8. As it mixes into the main system, it raises both calcium and alkalinity in proportion to coral consumption.

When a Calcium Reactor Is Worth It

Calcium reactors pay off economically in tanks over 75 to 100 gallons with significant SPS coral growth. Below that size, two-part dosing (BRS Two-Part Concentrate or ESV B-Ionic) is simpler and costs about the same or less. On a 180-gallon SPS tank, two-part cost can run $80 to $120 per month, while a calcium reactor uses roughly $20 in CO2 and $20 to $30 in media, saving $40 to $80 monthly.

  • Small reef tanks (50 to 100 gallons): Aquamedic Carbo Calc 1.0, Two Little Fishies Kalkreactor 200
  • Medium reef (100 to 200 gallons): Reef Octopus MF-800, Deltec PF601, BRS Single Reactor
  • Large reef (200+ gallons): Deltec PF-1001, Reef Octopus MF-1000, MTC Pro-Cal

For a detailed comparison of calcium reactor models, see the Best Aquarium Equipment guide which covers supplementation equipment across all reef tank sizes.

Phosphate Reactors (GFO Reactors)

A phosphate reactor tumbles granular ferric oxide (GFO) or similar iron-based media in a slow flow of tank water. GFO binds phosphate ions from the water, removing them from the system. This keeps phosphate below 0.05 ppm, which is important for preventing nuisance algae and allowing coral to grow without inhibition.

How They Work

Water enters the reactor from the bottom, flows slowly up through the GFO media bed, and exits from the top. The flow rate is set just high enough to gently tumble the media (you should see slow, constant movement of the media inside the clear chamber). Too fast and the media grinds itself into dust; too slow and it compacts into a solid block that water channels through rather than filtering.

Flow Rate for GFO Reactors

Most media manufacturers specify a target flow rate. For BRS High Capacity GFO, the recommended rate is 1 to 2 bed volumes per minute. A bed volume is the volume of media in the chamber. For 250 mL of GFO, that's 250 to 500 mL per minute, or roughly 4 to 8 gallons per hour. This is much slower than filtration pumps, so a small dedicated powerhead or a flow restrictor on the sump pump line is required.

The BRS Dual Reactor (runs carbon and GFO simultaneously in separate chambers) is a popular choice at around $60 to $80. The Bashsea Single Reactor is another reliable option.

When to Run GFO

Run GFO if phosphate tests above 0.05 ppm on a reef with LPS or SPS corals, or if you're seeing nuisance algae growth that correlates with phosphate. Replace GFO when it's exhausted (media turns from orange-brown to dark brown/black and phosphate rises again), typically every 4 to 8 weeks depending on tank load. Introduce GFO slowly by starting with half the recommended amount for two weeks, since a rapid phosphate drop can stress corals adapted to higher levels.

Carbon Reactors

A carbon reactor tumbles activated carbon in a water flow to improve chemical filtration. Activated carbon removes tannins, organic compounds, medications, volatile organics, and yellowing compounds from the water. This improves water clarity and removes compounds that would otherwise accumulate between water changes.

Activated Carbon vs. Passive Carbon Bags

Carbon bags placed in the sump work passively but less efficiently than a reactor because water flow through a loose bag is uneven. A reactor ensures all water contacts the carbon evenly. For a system with a high organic load (heavy feeding, high fish population), a carbon reactor provides noticeably better water quality than a passive bag.

How Long Carbon Lasts in a Reactor

In a heavily loaded reef tank, activated carbon typically exhausts in 2 to 4 weeks. Lighter loads extend this to 4 to 8 weeks. Signs of exhaustion: water yellowing, slight odor, or a visible drop in water clarity. Two Little Fishies HydroCarbon and Seachem Matrix Carbon are widely used reactor-grade carbons.

Running Carbon and GFO Together

The BRS Dual Reactor is designed specifically for this, running GFO in one chamber and activated carbon in the other from a single flow source. This simplifies plumbing and is the standard setup in most intermediate to advanced reef systems.

Nitrate Reactors (Sulfur Denitrators)

A nitrate reactor (or sulfur denitrator) is a specialized reactor that runs under nearly zero-flow conditions, creating an anaerobic environment where bacteria use sulfur media as an energy source to convert nitrate to nitrogen gas. This is one of the few ways to actively reduce nitrate without water changes or protein skimmers.

How Sulfur Denitrators Work

Water flows through the reactor extremely slowly, typically 2 to 4 drops per minute. At this flow rate, bacteria deplete the available oxygen inside the chamber, creating an anaerobic zone. Sulfur-reducing bacteria (Thiobacillus denitrificans) oxidize sulfur compounds while reducing nitrate to nitrogen gas, which off-gasses harmlessly. The effluent from a working sulfur denitrator has nearly zero nitrate and a slightly sulfurous smell.

Calibration Challenges

Sulfur denitrators are finicky to calibrate. Too fast a flow and the chamber stays aerobic and doesn't reduce nitrate. Too slow and the effluent becomes acidic and can introduce hydrogen sulfide. Calibration takes weeks of fine adjustment. Most hobbyists consider sulfur denitrators a specialty item suited to experienced reef keepers who need nitrate reduction beyond what water changes and protein skimming provide.

The Coralife BioCube Denitrator and the Aqua Medic Nitratreductor are common models. More advanced hobbyists build their own from clear acrylic tube with an adjustable valve.

Biopellet Reactors

Biopellet reactors are an alternative method for nitrate and phosphate reduction that's become popular since the early 2010s. They tumble a solid carbon source (biopellets, usually a polylactic acid polymer) in a fast flow of water. Bacteria colonize the pellets and consume the carbon along with nitrate and phosphate, then the bacteria-rich water is exported by the protein skimmer.

Key Requirement: Aggressive Skimming

Biopellet reactors only work well if paired with an efficient, well-sized protein skimmer. The bacteria produced need to be physically removed, not left to decay in the sump. Without a good skimmer, biopellets can cause bacterial blooms and drop oxygen levels.

BRS Biopellet Reactor and Two Little Fishies NPX Bioplastics

The BRS Biopellet Reactor is the most commonly used unit, paired with BRS biopellets or Two Little Fishies NPX Bioplastics media. Use one cup (roughly 250 mL) of biopellets per 100 gallons of tank water as a starting amount, adjusting after 4 to 6 weeks based on test results.

For a complete overview of top marine equipment options and how reactors fit into a full reef system, the Top Aquarium Equipment guide covers skimmers, reactors, and supplementation equipment together.

FAQ

Which marine tank reactor should I buy first? For most reef tanks, a GFO/carbon dual reactor like the BRS Dual Reactor is the most broadly useful first purchase. It addresses phosphate buildup and organic compound removal simultaneously, benefits tanks with any coral type, and costs $60 to $80. A calcium reactor comes next if you have significant SPS coral growth and are spending $50+ monthly on two-part dosing chemicals.

Can I run a calcium reactor and biopellet reactor at the same time? Yes, and many reef keepers do. The calcium reactor addresses calcium and alkalinity; the biopellet reactor addresses nitrate and phosphate biologically. They don't interfere with each other, though running both does require a well-sized protein skimmer and careful monitoring to avoid imbalances.

Do I need CO2 equipment for a phosphate or carbon reactor? No. CO2 is only required for calcium reactors. GFO reactors, carbon reactors, biopellet reactors, and nitrate reactors all use water flow alone. CO2 is specific to calcium reactors because the acidification it provides is what dissolves the calcium carbonate media.

How do I know which reactor problems to troubleshoot first? Run a full water parameter test: calcium, alkalinity, phosphate, nitrate, and pH. The failing parameters point directly to which reactor to look at. Low calcium and alkalinity together point to the calcium reactor's effluent rate or CO2 settings. High phosphate despite GFO running points to exhausted media or incorrect flow. High nitrate despite a biopellet reactor suggests the reactor is undersized or the skimmer isn't removing export efficiently.