Biopellet Reactor Reef Tank

A biopellet reactor is a specific type of bio reactor that uses solid polymer pellets as a slow-release carbon source to fuel bacteria-driven denitrification. In a reef tank, that means the reactor helps bring nitrates and phosphates down by growing a dense bacterial colony that consumes these compounds as it processes the pellets. Most reef keepers who add one see measurable nitrate reduction within 3 to 5 weeks, and many reach near-zero nitrates within 2 months.

This guide covers how biopellet reactors work mechanically and biologically, how to pick the right one for your tank size, setup best practices, what to expect during the break-in period, and the most common mistakes people make.

How Biopellet Reactors Work

Biopellets are made from polyhydroxyalkanoates (PHA), a type of biodegradable plastic produced by bacteria in natural environments. In the reactor, bacteria colonize the pellet surface and gradually break them down for carbon, using that energy to also consume dissolved nitrates and phosphates from the surrounding water.

The reactor itself is a sealed cylindrical chamber. Water is pumped in from the bottom, flows upward through the pellet bed, tumbles the pellets to keep them from compacting, and exits at the top. The tumbling is essential: pellets that sit still develop anaerobic zones inside the pile, which can produce hydrogen sulfide and crash your system's water quality.

The bacterial biomass that builds up gets exported by your protein skimmer. This is why routing the reactor outlet directly to the skimmer inlet (not back into the sump) is non-negotiable. Without the skimmer removing excess bacterial growth, you'd see a bacterial bloom in the water column.

What the Pellets Actually Do

Biopellets release carbon slowly as they dissolve. This steady carbon release is the main advantage over liquid carbon dosing (vodka, vinegar, or commercial products). With liquid dosing, you're adding carbon in discrete daily doses that can cause bacterial spikes if the dose is wrong. Pellets provide a near-continuous supply that the bacteria regulate themselves to some extent.

Two Little Fishies NP BioActif Pellets and Brightwell Aquatics Xport-NO3 are the two most widely used products. Both contain PHA pellets but differ slightly in density and dissolution rate. Royal Nature also makes a well-regarded pellet. Most hobbyists don't notice a meaningful performance difference between brands at comparable dosing volumes.

Choosing the Right Biopellet Reactor

For Tanks Under 100 Gallons

The Two Little Fishies PhosBan Reactor 150 is the most popular entry-level option. It's designed for media reactors but works for biopellets when paired with the right pump. The body holds up to about 150ml of pellets comfortably. Running it with a Cobalt MJ1000 or Tunze 1073.008 gives you the right flow for gentle tumbling.

The BRS Dual Media Reactor in the smaller configuration is another widely used option. BRS sells it as a kit with the pump included, which simplifies the setup.

For Tanks 100 to 250 Gallons

The Deltec FR509 is a step up in capacity, holding up to 500ml of pellets and rated for tanks to about 250 gallons. It uses a quality Askoll pump included in the package.

The Reef Octopus BM 150 is similarly sized and uses a good pump. Reef Octopus has a well-earned reputation for build quality in their reactor line.

For Large Tanks Over 250 Gallons

The Deltec FR1509 and the Vertex Libra series handle large volumes. Some reefers with 500+ gallon systems run two smaller reactors in parallel rather than one very large one, since multiple smaller reactors are easier to manage and tune independently.

Setting Up Your Biopellet Reactor: Step by Step

Step 1: Rinse the Pellets

Before loading, rinse biopellets in RO/DI water for 30 seconds to remove dust and manufacturing residue. You'll see cloudy water run off initially. This is normal.

Step 2: Load and Plumb

Add pellets to the reactor chamber. Fill to no more than 70% of the chamber volume to leave room for tumbling. Connect the pump inlet to your sump and the reactor outlet to your protein skimmer's intake.

Step 3: Set Initial Flow Rate

Start with a flow rate that causes the pellets to slowly and gently swirl. They should move, not be violently churned. Over-aggressive flow can break pellets into fine powder that passes into the skimmer and clogs the pump impeller.

Step 4: Monitor During Break-In

Check the reactor daily for the first week. Verify pellets are tumbling, confirm no pellet slurry is passing to the skimmer, and watch skimmer output. By week 2 to 3, skimmer production usually increases noticeably. By week 4 to 6, nitrate readings typically begin declining.

Break-In Period: What to Expect

Week 1 to 2: Essentially no measurable effect on nitrate or phosphate. Bacterial colonization is just beginning.

Week 2 to 4: Skimmer starts producing more and darker skimmate. This is the bacterial biomass being exported. You may see slight nutrient decline.

Week 4 to 8: Nitrate decline becomes measurable. Phosphate response often follows nitrate by 1 to 2 weeks.

Week 8 and beyond: Most tanks with properly dosed biopellets reach target nitrate levels (below 5 ppm for most reef tanks, or 1 to 2 ppm for SPS-dominant systems).

Test every 7 to 10 days during this period and record the results. This gives you data to adjust pellet volume intelligently rather than guessing.

Maintenance

Monthly: Check pellet volume. Pellets shrink as they're consumed. Top up to your target volume when they've decreased by about 25%.

Every 2 to 3 months: Remove the pump, disassemble the impeller housing, and rinse out any accumulated biofilm. A clogged impeller delivers inconsistent flow, which can cause pellet compaction.

Every 6 to 12 months: Replace the full pellet batch when volume has decreased by 50%+ or when the reactor starts becoming less effective despite correct pellet volume. Old pellets become denser and colonization drops off.

For equipment recommendations including protein skimmers to pair with your biopellet reactor, our best aquarium equipment guide covers options at multiple price points.

Common Mistakes

Running the reactor without a skimmer. You cannot effectively run biopellets without a protein skimmer. The bacterial export mechanism doesn't work otherwise, and you'll end up with a bacterial bloom in your display tank.

Adding too many pellets too quickly. Start at the low end of recommended dosing. Aggressive bacterial growth can crash oxygen levels and cause a bacterial snowstorm in your tank. Add pellets in 50ml increments over several weeks.

Not routing the output to the skimmer inlet. Running reactor output back into the sump and expecting the skimmer to catch it downstream is less effective. Route the output tubing directly to the skimmer's water inlet for best export.

Ignoring flow rate. Pellets that aren't tumbling are compacting. Check the reactor visually every 3 to 4 days. If the pile has settled and isn't moving, increase flow slightly.

More troubleshooting and equipment pairing advice is available in our top aquarium equipment roundup.

FAQ

Will biopellets cause problems in a lightly stocked tank? They can. In a lightly stocked tank with already low nutrients, biopellets can strip nutrients too aggressively, leaving your tank with nitrates under 1 ppm. While that sounds ideal, some corals (particularly LPS and softies) grow and color up better with 2 to 5 ppm nitrates. Monitor levels closely and reduce pellet volume if nutrients drop too low.

Can I run biopellets with a refugium? Yes, and this combination is effective. The refugium handles phosphate through algae uptake, and the biopellets address nitrate through bacterial denitrification. Run them simultaneously and monitor both parameters to balance the two systems.

How do I know my reactor flow is correct? Correct flow causes pellets to gently tumble in a circular pattern throughout the chamber. Too little flow: pellets sit in a pile. Too much flow: pellets shoot to the top and bunch up, or fine particles pass through. Adjust the pump output valve until you see gentle, even tumbling.

Are biopellets better than a refugium with chaeto? Both work, and they excel at different parameters. Chaeto is excellent at stripping phosphate and works well in tanks with moderate nitrate. Biopellets handle nitrate very effectively but are somewhat less consistent on phosphate. Many reefers run both. If you only have room for one, the choice depends on whether nitrate or phosphate is your bigger problem.

Final Thoughts

A biopellet reactor is a reliable, relatively hands-off way to manage nitrates in a reef tank. Get the sizing right for your tank volume, route the outlet to your skimmer, and be patient during the 4 to 8 week break-in period. Once established, the maintenance routine is straightforward: monthly pellet top-ups and occasional pump cleaning. For tanks with persistent nitrate problems that water changes alone aren't solving, biopellets are often the most effective next step.