Marine Aquarium Reactors

Marine aquarium reactors are sealed chambers that pass aquarium water through a media or substance under controlled flow conditions to achieve a specific water chemistry result. The most common types are calcium reactors, carbon/GFO reactors, biopellet reactors, and refugium-style media reactors. Each addresses a different problem in reef keeping, and understanding which reactor does what helps you decide which ones your system actually needs.

Reactors are specialized tools, not universal requirements. A fish-only saltwater tank rarely benefits from any of them. A densely stocked SPS reef tank may run three or four simultaneously. This guide covers each major reactor type, how they work, which brands are worth using, and how to set them up correctly.

Calcium Reactors

A calcium reactor is the most sophisticated and impactful reactor type for reef keeping. It maintains calcium and alkalinity in a reef tank by dissolving calcium-carbonate media (typically aragonite or coralline rubble) using CO2 acidification.

How a Calcium Reactor Works

Water enters the reactor chamber where it mixes with CO2 gas. The CO2 drops the pH inside the reactor (usually to around 6.3 to 6.7), which creates a mildly acidic environment that dissolves the aragonite media. This dissolution releases calcium and carbonate ions into the effluent water, which drips back into the sump and raises calcium and alkalinity levels in the tank.

The key adjustments are CO2 bubble rate and effluent drip rate. More CO2 increases dissolution speed. Higher effluent rate increases how quickly the enriched water returns to the tank. Dialing in these two parameters maintains target calcium (380 to 450 ppm) and alkalinity (8 to 12 dKH) levels without manual dosing.

Calcium Reactor Models

The Two Little Fishies Nano Reactor 150 handles tanks up to 75 gallons and is a well-regarded compact option at around $200. The Reef Octopus CaRx 150 is a step up in build quality with better bubble control for tanks up to 150 gallons. For larger systems, the GEO Calcium Reactor 618 and the Deltec PF601 have been used reliably by experienced reef hobbyists for years, though their pricing reflects serious reef setups.

Calcium reactors require a CO2 cylinder and a regulator to supply CO2. A 5-pound CO2 cylinder lasts roughly four to eight months depending on bubble rate and tank size. CO2 regulators with solenoids (the solenoid closes the CO2 when the controller pH drops too low) prevent pH swings in the tank from CO2 overdose.

For detailed coverage of calcium reactors specifically, the best calcium reactor for reef tank guide and the best CO2 regulator for calcium reactor review are worth reading before purchasing.

Carbon and GFO Reactors

These reactors run activated carbon and/or granular ferric oxide (GFO) through a fluidized media bed, which keeps the media in constant movement for better contact efficiency.

How Media Reactors Work

Water enters the bottom of a clear cylinder containing media, flows up through the media bed, and exits from the top. The flow rate is adjusted so the media "fluidizes," tumbling slowly without being so vigorous that it grinds the media down prematurely.

Activated carbon in a reactor removes dissolved organics, yellowing compounds, and odors from the water. Running carbon keeps water crystal clear and polishes out dissolved waste that skimming misses. Changing carbon every four to six weeks is the standard schedule.

GFO removes phosphate from the water column. High phosphate (above 0.05 ppm in a reef tank) fuels nuisance algae and can inhibit coral calcification. GFO reactors maintain phosphate at or near zero, which is the target for most reef systems.

Carbon/GFO Reactor Models

The Two Little Fishies Phosban 150 is the classic small media reactor, handling tanks up to 100 gallons for around $55. It's simple, reliable, and the most commonly recommended entry-level reactor.

The Aqua Maxx FR-F Series (FR-F1, FR-F2) offers higher capacity options with clear acrylic bodies that let you inspect media condition easily. The BRS Single Reactor is a budget-friendly option from Bulk Reef Supply that works for basic GFO and carbon applications.

For tanks with high phosphate problems, running a two-chamber reactor (carbon in one chamber, GFO in another) allows both to operate simultaneously without mixing.

Biopellet Reactors

Biopellet reactors contain solid carbon pellets that beneficial bacteria colonize. As bacteria consume the pellets, they also consume nitrate and phosphate from the water passing through, effectively exporting nutrients through bacterial biomass.

When Biopellets Make Sense

Biopellets work best in tanks with excess nitrate from heavy feeding or high fish density, where other nutrient export methods (skimming, water changes) aren't keeping up. They're particularly common in heavily stocked FOWLR tanks and fish-only systems.

Biopellets don't replace skimming. The bacterial biomass that develops on the pellets needs to be exported by a protein skimmer. Running biopellets without a skimmer creates bacterial waste that stays in the water column and causes other problems.

The Innovative Marine Aquaculture Nuvo Fusion reactors and the BRS Mini Biopellet Reactor are standard choices for this application. Starting with a small quantity of pellets and increasing gradually prevents bacterial blooms that can crash oxygen levels.

Refugium Reactors and Macroalgae Chambers

A refugium is a separate small tank or chamber where macroalgae (typically chaeto, or Chaetomorpha) grows under a dedicated light. As the algae grows, it uptakes nitrate, phosphate, and other nutrients. Harvesting the algae exports those nutrients from the system.

This functions as a reactor in the sense that water cycles through the refugium chamber at a controlled rate. Unlike the mechanical reactors above, a refugium is simply a plant-based nutrient export method housed in an auxiliary chamber.

The Innovative Marine NUVO Sump includes a refugium section. For hang-on-back options, the CPR Aquafuge Pro provides a small refugium without requiring a sump. Refugium lights like the Kessil H380SE or the Tunze LED 0093 are designed specifically for chaeto growth spectrum requirements.

Setting Up a Reactor: General Principles

Regardless of reactor type, setup follows a consistent process.

Flow rate. Every reactor specifies an inlet flow rate for proper operation. Too little flow and the reactor doesn't process enough water volume. Too much and the media doesn't have contact time to be effective. A small dedicated pump or a tee off your return line with a ball valve to control flow handles this.

Plumbing. Most reactors use 3/8-inch or 1/2-inch barbed fittings with flexible tubing. Quick-disconnect fittings make maintenance easier. Locate the reactor in the sump cabinet where it's accessible for media changes and adjustments.

Monitoring. After starting a reactor, test the target parameter (calcium, alkalinity, phosphate) every few days and adjust flow and CO2 (for calcium reactors) until the parameter stabilizes at target levels. This dialing-in period takes one to two weeks for most systems.

The best aquarium equipment page covers where reactors fit within the broader reef equipment hierarchy and what to prioritize when building out a new system.

Do You Need Reactors?

The honest answer is: it depends on tank size and livestock density.

A 40-gallon reef tank with a moderate number of LPS corals can be maintained successfully with a protein skimmer, occasional manual dosing of two-part solutions, and regular water changes. No reactors required.

A 150-gallon SPS-dominant system where you're adding 50 to 100 corals and running high flow benefits significantly from a calcium reactor for stable parameter maintenance, a GFO reactor for phosphate control, and possibly a carbon reactor for water clarity. The parameter stability that reactors provide at scale is difficult to replicate manually.

The top aquarium equipment for serious reef keeping almost always includes at least a calcium reactor and a media reactor for GFO once a system reaches a certain coral density.

FAQ

Do I need a calcium reactor for a small reef tank? Not necessarily. Two-part dosing (adding calcium and alkalinity supplements separately) handles smaller tanks well and costs less to set up. A calcium reactor becomes cost-effective and more practical than manual dosing around the 75 to 100-gallon mark and above.

Can I run carbon and GFO in the same reactor chamber? It's not recommended. Carbon can exhaust prematurely next to GFO and the two media have different ideal flow rates. Running them in separate chambers (or a two-chamber reactor) prevents these issues.

How long does reactor media last? Aragonite in a calcium reactor lasts six to eighteen months depending on dissolution rate and reactor volume. GFO needs changing every four to eight weeks. Activated carbon every four to six weeks. Biopellets slowly consume over several months; top them up periodically.

Is a media reactor better than a media bag in a sump? For carbon, a bag in the sump works reasonably well. For GFO, a fluidized reactor significantly outperforms a static bag because the moving media has more surface area contact with water and doesn't channel. Phosphate removal with a bag is about 30 to 40 percent less efficient than the same amount of GFO in a properly set up reactor.

Summary

Marine aquarium reactors are tools for precision chemical management. Calcium reactors maintain coral building blocks at scale. Media reactors keep phosphate and dissolved organics in check. Biopellet reactors export nutrients in high-stocking situations. Start with a media reactor for phosphate control if that's a problem, add a calcium reactor when your coral density outgrows manual dosing, and layer in others based on specific needs rather than general recommendations.