Reactor media is the filtering material you load inside a media reactor to remove specific compounds from your aquarium water. The reactor itself is just the vessel; the media is what does the actual work, and choosing the right type makes or breaks the system's performance. Whether you're running GFO to strip phosphates, activated carbon to kill dissolved organics, or biopellets to drive biological nutrient reduction, the media category determines exactly what you're solving for.
This guide covers the main types of reactor media used in reef and freshwater tanks, how each one works, how to size your media load, and the mistakes that trip up new reactor users. By the end you'll know what to buy, how much to run, and how to avoid the most common pitfalls.
The Main Types of Aquarium Reactor Media
Not all reactor media is interchangeable. Each type targets a different problem and needs specific flow conditions to work properly.
GFO (Granular Ferric Oxide)
GFO is the most widely used reactor media in reef keeping. Iron oxide beads adsorb phosphate out of the water column through chemical binding. The phosphate molecule attaches to the ferric oxide surface and stays there until you discard the spent media.
Two phosphate levels are used as benchmarks: most reef keepers target 0.03 ppm or below for SPS corals, and up to 0.10 ppm for mixed or LPS tanks. GFO can drive phosphate down from elevated levels (0.5 ppm+) to near zero within 48 to 72 hours when dosed correctly.
Popular options include Two Little Fishies PhosBan, BRS Bulk GFO, and Seachem PhosGuard. BRS High Capacity GFO is widely used because it lasts longer than standard GFO before exhaustion.
The amount you run matters a lot. BRS recommends starting with half the standard dose (roughly 1/4 cup per 100 gallons for standard GFO) and doubling only if phosphate doesn't drop after one week. Overdosing GFO too fast can cause a rapid phosphate crash, which stresses corals that have adapted to elevated levels.
Activated Carbon
Carbon is the workhorse for water clarity and dissolved organic removal. It adsorbs yellowing compounds, dissolved proteins, some medications, and volatile organics that cause musty odors.
Pelletized carbon (like BRS Rox 0.8 Carbon or Two Little Fishies ROX 0.8) is the standard choice for reactors because loose granular carbon can channel and leave dead spots. In a reactor with gentle tumbling, pelletized carbon contacts water evenly.
Change carbon every 4 to 6 weeks. After that point the surface area is exhausted and the carbon stops removing compounds. Leaving it longer doesn't harm the tank directly, but you're getting no benefit from the reactor running.
Biopellets
Biopellets are a biological media made from a biodegradable polymer (usually polylactic acid). Bacteria colonize the pellet surface and consume nitrate and phosphate as they break down the polymer. The bacteria-laden pellets then get skimmed out of the system.
Running biopellets requires a protein skimmer. Without one, the bacterial bloom you generate has nowhere to go and can cause water quality problems. Also, biopellets need vigorous tumbling (not gentle flow) to prevent clumping and anaerobic pockets.
Two Little Fishies NPX Bioplastics
NPX pellets work similarly to biopellets but are designed for gentler tumbling and are often considered easier to dial in for beginners. They're a solid middle-ground option if you want biological nutrient reduction without the high-flow demands of standard biopellets.
Zeolites
Zeolites bind ammonia and are used primarily in freshwater setups or in the zeovit method for ultra-low-nutrient reef tanks. In a zeovit system, the zeolite reactor is shaken daily to slough bacteria off the media surface, which a skimmer then removes.
How to Choose Reactor Media for Your Setup
The right media depends entirely on what your test results show.
If phosphate is your main issue: start with GFO. If both nitrate and phosphate are elevated: consider biopellets or a carbon dosing regimen (two-part or vodka dosing) alongside GFO. If water clarity is the goal: activated carbon. If you're running a zeovit system: zeolites are part of that specific methodology and work alongside their additive line.
You can run multiple media types simultaneously by using two reactors in line or a dual-chamber reactor. A common configuration is a two-stage reactor where the first chamber runs GFO and the second runs activated carbon.
For product recommendations, our guide to the best media reactor for reef tank covers the reactor units themselves so you can match them to the media you've chosen.
How Much Reactor Media to Use
Starting dosing guidelines for common media:
- GFO (standard grade): 1/4 cup per 100 gallons of system volume, adjusted based on weekly phosphate tests
- GFO (high capacity): 1/8 cup per 100 gallons to start; high capacity media has more surface area so it's stronger per volume
- Activated carbon: 1/4 to 1/2 cup per 50 gallons, changed every 4 to 6 weeks
- Biopellets: 1 mL per gallon is a common starting point; scale up slowly over 4 to 6 weeks
- NPX bioplastics: Similar to biopellets; follow manufacturer label and scale up based on nutrient readings
These are starting points, not final numbers. Weekly testing drives your adjustments. The goal is stable, gradual improvement, not a rapid crash in either direction.
Flow Rate and Tumbling: Why They Matter
Media reactors fail when flow is set wrong. Each media type needs different conditions.
GFO should tumble gently. You want to see the media slowly rotating, not spinning hard. Hard tumbling breaks GFO into dust, which can escape into the display tank as a brown cloud. Most reefers run GFO at a flow rate that creates slow, continuous movement.
Activated carbon in pellet form needs even slower flow. The goal is just enough water movement to ensure contact with the media, not vigorous tumbling.
Biopellets need vigorous tumbling to keep them from clumping. A clumped mass of biopellets creates anaerobic zones that can crash water quality. If you see the pellets sticking together, increase flow rate until they're actively tumbling.
Most reactors have a flow adjustment valve on the input side. Start low, observe for 30 minutes, and dial up as needed.
Common Mistakes With Reactor Media
Running media too long. GFO and carbon both exhaust. Testing your water and seeing no improvement after 2 to 3 weeks is often a sign the media is spent, not that the reactor isn't working.
Not testing before and after adding media. You can't know if the reactor is working without a baseline. Test before you start and again at 3, 7, and 14 days.
Overdosing GFO in a tank with stable corals. If your corals have adjusted to phosphate at 0.2 ppm and you crash it to 0.01 ppm in 48 hours, expect stress or bleaching. Start with a half dose and extend the adjustment period to 2 to 3 weeks.
Using the wrong media type for the reactor. Zeolite reactors need daily agitation. Biopellet reactors need specific flow. Using GFO in a biopellet-style reactor with high flow will result in GFO dust everywhere. Match the reactor design to the media.
Skipping carbon while running biopellets. Biopellets increase dissolved organic load in the water before the skimmer removes it. Running carbon alongside biopellets keeps water clarity up and handles organics that escape skimming.
For a broader look at the equipment involved, our best aquarium equipment roundup covers reactors, skimmers, and the other gear that supports your filtration system.
When to Replace Reactor Media
Replace GFO when phosphate stops dropping despite maintaining the correct dose and flow. In a typical reef tank with moderate bioload, GFO lasts 4 to 8 weeks. High bioload tanks may exhaust it in 2 to 3 weeks.
Activated carbon should be swapped every 4 to 6 weeks regardless of appearance. Spent carbon looks identical to fresh carbon; you can't tell by looking at it.
Biopellets replenish slowly as they're consumed. Add fresh pellets every 2 to 3 months rather than doing a complete swap; topping up maintains the established bacterial colony.
Zeolites are exchanged every 6 to 8 weeks in the zeovit method.
Set a calendar reminder when you load fresh media. Without a date to work from, it's easy to leave exhausted media running for months.
FAQ
Can I mix GFO and carbon in the same reactor chamber? You can, but it's not ideal. GFO needs gentle tumbling and carbon needs slower flow. Mixing them in a single chamber means neither is running at its optimal flow rate. A dual-chamber reactor or two separate reactors in series is a better setup if you want to run both simultaneously.
How do I know my GFO is exhausted? Test phosphate weekly. When phosphate stabilizes at a level above your target despite the reactor running for 2 to 3 weeks, the GFO surface area is saturated. Discard and reload. There's no visual indicator on the beads themselves.
Do I need a media reactor for a small tank? No. For tanks under 30 gallons, media bags hung in the sump or placed in a high-flow area of the filter can work adequately. Reactors become more effective at larger volumes where you need consistent, controlled contact time between water and media. For small setups, the cost and plumbing complexity of a reactor often isn't worth it.
Can biopellets replace my protein skimmer? No. Biopellets require a skimmer to function. The biological process generates bacterial biomass that needs to be physically removed from the system. Without a skimmer, the bacteria just die in the water column and add organic load instead of removing it.
Wrapping Up
The media you choose for your reactor should match the specific water quality problem you're solving. GFO for phosphate, carbon for dissolved organics and clarity, biopellets for combined nitrate and phosphate reduction through biological means. Start with conservative doses, test weekly, and adjust based on results rather than guessing. That approach will get you to your target parameters without stressing your livestock.