A reef media reactor is a sealed cylinder that forces aquarium water through a bed of filtering media under controlled flow conditions. The controlled contact time between water and media is what makes reactors more effective than hanging a media bag in your sump. For most reef tanks running GFO, activated carbon, or biopellets at scale, a dedicated reactor is the right tool. For small setups under 30 gallons, a media bag often does the job without the plumbing complexity.
This guide explains how reef media reactors work, the different media types you can run through them, how to size and set up a reactor correctly, and which situations actually justify the investment. If you're deciding whether to buy one or wondering why your current reactor isn't working as expected, this covers the key questions.
How a Reef Media Reactor Actually Works
The principle is straightforward. Water enters the bottom of the reactor chamber, passes upward through the media bed, and exits through the top. A flow valve on the input line controls the speed of this movement.
What makes this better than a media bag is contact time and surface area coverage. In a media bag sitting in a sump, water flows around the bag rather than through it. The media inside contacts a fraction of the water passing through the system. In a reactor, all of the water flowing through the unit must pass through the media. Every gallon goes through, not around.
For GFO (granular ferric oxide), this consistent contact time is critical because phosphate removal is a surface-adsorption reaction. GFO removes phosphate by having phosphate molecules physically bind to the iron oxide surface. The more surface area that contacts water, and the more consistently water flows through, the more effective the removal rate.
For biopellets, the controlled flow maintains tumbling conditions that keep bacteria aerobic. Stop the tumbling and biopellets clump, creating anaerobic zones that produce hydrogen sulfide rather than removing nitrate.
External vs. In-Sump Reactors
External reactors mount outside the sump and use a small pump or a tee off the return line to feed water through them. Two Little Fishies PhosBan Reactor 150 and 550 are popular external designs. They're easy to access for media changes and don't occupy sump space.
In-sump reactors (like the Reef Octopus MRC-1 or similar designs) sit inside the sump and are powered by the return pump or a dedicated small pump. They're less visible but take up sump space.
For most hobbyists, external reactors are easier to work with because you can disconnect and service them without reaching into the sump.
Choosing the Right Media for Your Reactor
The media you run determines what your reactor accomplishes. Running the wrong media or the wrong amount is the most common reactor setup mistake.
GFO for Phosphate Control
GFO is the most common reef reactor media. It adsorbs phosphate through a chemical binding process and is the go-to solution for elevated phosphate in reef tanks. Target phosphate levels depend on what you're keeping: 0.03 to 0.05 ppm for SPS-dominant tanks, up to 0.10 ppm for mixed or LPS tanks.
Starting dose: 1/4 cup per 100 gallons of system volume for standard GFO. High-capacity GFO (BRS High Capacity, Two Little Fishies PhosBan) has more surface area per volume, so start with half that amount.
GFO exhausts over time (typically 4 to 8 weeks depending on bioload) and needs to be replaced rather than regenerated. When your phosphate stops dropping despite the reactor running, the GFO is spent.
Activated Carbon for Water Clarity
Carbon removes dissolved organics, yellow tannins, and some medications from the water column. Pelletized activated carbon (Two Little Fishies ROX 0.8 Carbon, BRS Rox 0.8) is better for reactors than granular carbon because it flows more evenly. Change carbon every 4 to 6 weeks.
Biopellets for Biological Nutrient Reduction
Biopellets drive bacterial growth that consumes nitrate and phosphate. The bacteria colonize the polymer pellet surface, and a protein skimmer removes the bacterial biomass from the system. Biopellets require vigorous tumbling (not gentle flow) and a running protein skimmer. Without a skimmer, biopellets don't work.
For our recommendations on specific reactor units that handle all three media types, see the best media reactor for reef tank guide.
Sizing a Reactor to Your Tank
Reactors are sized by the volume of media they hold, which you match to your tank's gallonage.
A common reactor sizing guide:
| Tank Size | GFO Load | Recommended Reactor |
|---|---|---|
| Up to 75 gallons | 1/2 to 3/4 cup | BRS Dual Reactor Small, PhosBan 150 |
| 75 to 150 gallons | 3/4 to 1.5 cups | PhosBan 550, BRS Dual Reactor Large |
| 150 to 300 gallons | 1.5 to 3 cups | Two Little Fishies PhosBan 1500, TLF Phosban 150 in parallel |
| 300+ gallons | Multiple reactors or large commercial units | Aqua UV, Pentair, DIY builds |
Don't oversize your reactor dramatically. A reactor with too large a chamber and too little media will have channeling issues where water flows around the media instead of through it.
Setting Up Flow Rate Correctly
Flow rate is the most common tuning mistake. Each media type needs different conditions.
GFO: Gentle tumbling. The media should rotate slowly in the chamber. Vigorous tumbling grinds GFO into dust that escapes into the display as a brown cloud and clogs the outlet screen. Typical flow rates run 40 to 100 mL per minute for a medium-sized reactor.
Activated carbon (pelletized): Even slower than GFO. You want steady water movement for contact time, not tumbling. Pelletized carbon doesn't need to move; it just needs water flowing through it.
Biopellets: Vigorous tumbling is required. You should see the pellets actively moving in the chamber at all times. Static biopellet beds clump and go anaerobic, which is worse than not running biopellets at all.
To measure flow rate without a specialized gauge: place a container of known volume under the reactor outlet, count how long it takes to fill, and calculate. A 500 mL water bottle filling in 5 minutes equals 100 mL/min.
How Long Does It Take for a Reactor to Work?
GFO reactors typically show measurable phosphate reduction within 48 to 72 hours of startup. By day 7 you should see clear progress toward your target if the media dose is appropriate. By day 14 you should be near or at target if the dosage is correct.
If phosphate hasn't moved after 2 weeks: 1. Check flow rate is actually moving water through the media 2. Verify the media amount is correct for your tank size 3. Test your phosphate test kit against a known reference (expired reagents give false results) 4. Consider whether your bioload is simply too high for the current GFO dose
Biopellets take longer to establish, typically 4 to 6 weeks before you see significant nitrate reduction, because the bacterial colony needs time to colonize the pellet surface.
For a broader look at how reactors fit into your filtration system alongside skimmers, lighting, and flow equipment, see our best aquarium equipment guide.
Common Reactor Problems and How to Fix Them
Air bubble trapped in chamber: Most reactors have a small bleed valve at the top to purge air. Open it slightly until water drips out, then close. Air pockets cause channeling and poor media contact.
GFO dust in display tank: Flow rate is too high. Reduce input flow until tumbling is gentle. Also check that the outlet screen is intact and not torn.
Reactor losing flow over time: Media is compacting and restricting flow, or the outlet screen is clogged. Disconnect, rinse the media in saltwater, clean the screen, and reassemble. If it's GFO that's been running for 4+ weeks, this compaction often means it's exhausted and ready to replace.
Biopellets clumping: Increase flow rate. If they're still clumping at maximum flow, the pellets may have biofilm buildup that's making them sticky. Remove, rinse briefly in old tank water (not tap water), and reload.
FAQ
Do I need two separate reactors to run GFO and carbon at the same time? Not necessarily. Dual-chamber reactors like the BRS Two Little Fishies dual reactor let you run two media types in separate chambers with independent flow control. This is more efficient than two separate units. Single-chamber reactors can only run one media type at optimal flow.
Can I run a reactor without a sump? Yes, but it requires more creative plumbing. You'd need a small pump to feed water from the display tank into the reactor and return it. External hang-on-back reactors like the Two Little Fishies PhosBan 150 are designed for exactly this setup and come with a small feed pump.
How often do I need to replace reactor media? GFO: every 4 to 8 weeks, or when phosphate stops dropping. Carbon: every 4 to 6 weeks on a schedule. Biopellets: top up every 2 to 3 months as they're consumed; a complete replacement isn't usually necessary if the colony is healthy.
Is a reactor better than chemical dosing for phosphate control? A reactor with GFO is more precise and controllable than chemical dosing for phosphate. Lanthanum chloride and similar liquid phosphate removers work but are easy to overdose, causing rapid crashes and coral stress. A reactor lets you control the reduction rate by adjusting flow and media dose, making it easier to lower phosphate gradually.
Final Thoughts
A reef media reactor is a precision tool for a specific job. For phosphate control in a reef tank, GFO in a properly sized reactor is the most reliable method available to hobbyists. Set the flow rate correctly for your media type, start with a conservative dose, test weekly, and replace media on schedule. Those four habits will get more performance out of your reactor than any equipment upgrade.