An in-sump media reactor is a sealed cylindrical chamber installed inside your aquarium sump that circulates water through a specific filtration media under controlled flow. Unlike passive media bags sitting loose in a sump baffle, a media reactor ensures every gallon of water makes contact with the media rather than channeling around it, which dramatically improves media efficiency. You get better performance from less media, and you get it more predictably.
In-sump reactors are most commonly used with GFO (granular ferric oxide for phosphate removal), activated carbon, and biopellets. They're standard equipment on serious reef tanks. Freshwater planted tanks sometimes use them for carbon or specialized media like Purigen. This guide covers how in-sump reactors work, which media they're best suited for, specific product options, and how to dial in flow rate for effective operation.
How a Media Reactor Works
The reactor body is a clear acrylic or polycarbonate cylinder, typically 2-4 inches in diameter and 8-18 inches tall. A small pump draws water from the sump into the bottom of the reactor, pushes it up through the media bed, and returns it to the sump.
The key design feature is the sealed chamber. All water entering must pass through the media; there's no path around it. This is called "plug flow" and it's significantly more efficient than media bags, where water flows around rather than through the media.
Media sits in the reactor as a bed of particles. The flow rate is adjusted so that water moves through the bed slowly enough for contact time but fast enough to gently fluidize the bed (keep it lightly tumbling rather than compacted into a solid cake). Compacted media has reduced effective surface area; media that tumbles too vigorously wears out faster and produces fine particles that escape the reactor.
Types of Media and Their Ideal Reactors
GFO (Phosphate Removal)
GFO removes phosphate from water by adsorption. High phosphate in reef tanks causes algae blooms and inhibits coral calcification. GFO in a passive media bag often clumps into a solid mass and processes only a fraction of its rated capacity. In a reactor, the gentle tumbling prevents clumping and keeps full surface area exposed.
Two-Little-Fishies Phosban 150 Reactor and the BRS Bulk Reef Supply Dual Reactor are the most commonly used GFO reactors. The Phosban 150 handles tanks up to about 150 gallons and uses a small powerhead (often the supplied Maxi-Jet 400) to pump water through the chamber.
Target flow for GFO: just enough to barely fluidize the bed. You want individual granules moving slightly but not tumbling vigorously. Start with the pump at minimum and increase until you see slight movement throughout the media.
Activated Carbon
Activated carbon removes organic compounds, tannins, dissolved medications, and some odors. In a reactor, carbon is fluidized gently with a slightly higher flow rate than GFO since carbon particles are denser.
Two Little Fishies Phosban Reactor works for carbon. The BRS Single Reactor or the Aquamaxx FR-SE fluidized reactor are other options that work well for either GFO or carbon depending on which media you load.
Change activated carbon every 4-6 weeks. Spent carbon doesn't just stop working; it can leach some adsorbed compounds back into the water if left too long.
Biopellets
Biopellets are biodegradable polymer pellets that serve as a carbon source for beneficial bacteria, which consume nitrate and phosphate as part of their metabolism. The bacteria colonize the pellets and need moderate fluidization to prevent dead spots in the bed and allow bacterial growth on all pellet surfaces.
Biopellet reactors require more vigorous fluidization than GFO reactors. The TLF Nano Biopellet Reactor and the Reef Octopus OCTO Biopellet Reactor are purpose-built for this. Run them with the reactor output pointed directly at the protein skimmer intake so that the bacterial sludge produced gets skimmed out immediately rather than entering the display.
A common mistake with biopellets is overloading the reactor (using too many pellets for your tank size). Start with half the rated amount. BRS recommends 100-150 mL per 100 gallons for a moderately stocked reef.
In-Sump vs. Hang-On-Back Reactors
Many media reactors are designed to hang on the outside of the sump (or display tank), with intake and return hoses running into the sump. In-sump reactors sit physically inside the sump with the pump submerged.
Advantages of In-Sump Placement
- No risk of a leak flooding the area around the sump
- Neater appearance since the reactor is inside the stand cabinet
- Reduced noise since the pump operates submerged
- Easier to adjust media level since you're working inside the sump
Advantages of Hang-On Placement
- Easier to monitor without opening the cabinet
- Reactor is accessible without reaching into the sump
- Can be added to setups that don't have a sump (hang on the display tank directly)
For most reef setups with an enclosed stand cabinet, in-sump placement is cleaner. For tanks without a sump or where cabinet space is limited, hang-on placement is more practical.
Specific Product Recommendations
Two Little Fishies Phosban 150
The Phosban 150 ($40-55) is arguably the most field-tested media reactor in the reef hobby. It's been continuously produced for over 15 years. The clear body lets you see media condition. It runs on a small powerhead (Maxi-Jet 400 or 600 depending on media type) and handles tanks up to 150 gallons for GFO.
One quirk: the inlet/outlet ports are at the bottom, which means installing it in a sump requires positioning it on a small platform or wedging it between baffle walls. It doesn't self-stand on the sump floor easily.
BRS Single Reactor
The BRS Single Reactor ($30-40) from Bulk Reef Supply is a simpler design with top and bottom caps that thread on and off for easy media changes. The inlet is at the bottom, outlet at the top, and it includes a small needle valve for precise flow adjustment. BRS also sells a dual reactor ($55-65) that runs two media types simultaneously from a single pump, which is useful when running both GFO and carbon.
The BRS reactor works with any pump that has a 1/2-inch output. Most hobbyists use the MJ 400 or Reef Octopus VarioS 2 submersible pumps.
Aquamaxx FR-SE Fluidized Media Reactor
The Aquamaxx FR-SE ($45-60) has a slightly larger body diameter than the Phosban 150 and a top-mounted inlet port, which makes it easier to position in a sump without a stand. The clear body and inch-marked side graduations make it easy to measure media volume. A good option if you're running it in a tight sump where positioning matters.
Reef Octopus OCTO 150 Biopellet Reactor
If biopellets are your primary application, the OCTO 150 ($60-80) is purpose-designed for the higher flow rates that biopellets require. Its internal circulation pattern produces consistent full-bed fluidization without channeling. It includes a dedicated output port with a directional nozzle to aim effluent at a skimmer intake.
Dialing In Flow Rate
Getting flow rate right is the most critical part of running a media reactor. Wrong flow rate means either compacted media (too slow) or media wearing down and escaping the reactor (too fast).
The visual test: watch media through the clear reactor body. GFO and carbon should barely move, just occasional gentle tumbling. Biopellets should tumble more actively but not shoot rapidly through the chamber.
If you can't see the media moving at all, increase flow. If media is visibly swirling in circular patterns faster than once every 3-4 seconds, slow down.
Most reactors use a needle valve between the pump and reactor to reduce flow from what the pump delivers to what the reactor needs. Operate the needle valve in small increments and give the reactor 5 minutes to stabilize after each adjustment before evaluating.
For a full rundown of reef filtration equipment including skimmers, reactors, and refugiums, see our guide to the Best Media Reactor for Reef Tank and Best Aquarium Equipment.
FAQ
Do I need both GFO and activated carbon in my reactor?
They address different problems. GFO removes phosphate; activated carbon removes organic compounds and improves water clarity. For most moderately stocked reef tanks, running activated carbon alone is sufficient if your feeding is controlled and phosphate stays below 0.05 ppm. Add GFO if phosphate climbs or you have persistent nuisance algae. A dual reactor lets you run both simultaneously without needing two separate reactors and pumps.
How much GFO should I use for my tank size?
BRS recommends starting with 1 cup (roughly 250 mL) of GFO per 100 gallons of system water for a lightly to moderately stocked reef. Increase if phosphate remains elevated after 2-3 weeks. Change GFO when it no longer reduces phosphate readings, usually every 4-8 weeks depending on bioload.
Can I run a media reactor on a freshwater planted tank?
Yes, activated carbon and Seachem Purigen are both commonly used in planted tank media reactors. Purigen is a synthetic adsorbent resin that controls ammonia, nitrite, and nitrate levels and can be regenerated with bleach solution when exhausted. The Two Little Fishies Phosban Reactor and BRS reactors work in freshwater applications without modification.
My reactor is producing fine white dust coming out the outlet. What's happening?
Fine particles of GFO or carbon are escaping through the outlet screen, which can clog filter socks and irritate fish gill tissue. This usually means the outlet screen is damaged or clogged in a way that's forcing water around it, or the flow rate is too high and physically grinding the media. Check the outlet screen for holes or tears. Reduce flow rate. If the media itself has broken down into fine powder, it's exhausted and needs replacement.