A bio reactor in a reef tank context is a sealed vessel that holds biological media and provides a controlled environment for bacteria to process waste products. The most common versions are carbon dosing reactors and biopellet reactors, both of which feed bacteria to encourage rapid denitrification. A bio reactor can bring nitrates and phosphates down from 20 to 40 ppm to near-zero within 4 to 8 weeks in a properly established reef system.
This guide explains the different types of bio reactors, how each one works, how to set them up, what to expect during startup, and how to troubleshoot common problems.
Types of Bio Reactors Used in Reef Tanks
Carbon Dosing Reactors
Carbon dosing reactors grow heterotrophic bacteria by providing a liquid carbon source. The bacteria consume the carbon and simultaneously strip nitrates and phosphates from the water. Vodka, vinegar, and proprietary products like Brightwell Aquatics NeoNitro or Two Little Fishies NoPox are all used as carbon sources.
The reactor vessel holds a sponge or mesh media for bacteria to colonize. Water flows through, the carbon source is dripped or pumped in at a controlled rate, and the enriched bacterial population processes waste aggressively.
Carbon dosing without a reactor is also practiced (just add the carbon source directly to the sump), but a reactor gives you more control over bacterial growth rate and prevents large population crashes from overdosing.
Biopellet Reactors
Biopellet reactors are the most popular bio reactor type in the reef hobby. They use solid polymer pellets made of biodegradable plastic (polyhydroxyalkanoates, or PHA). As water tumbles the pellets in the reactor, bacteria slowly break them down, using them as both a carbon source and a substrate for colonization.
The pellets provide a slow, controlled carbon release that's more forgiving than liquid dosing. You can't accidentally spike the carbon dose by adding too many pellets at once (within reason), whereas a miscalculated vodka dose can crash oxygen levels overnight.
Popular biopellet products include Two Little Fishies NP BioActif Pellets, Brightwell Aquatics Xport-NO3, and Royal Nature NP Biopolymers. The pellets last several months to over a year depending on your bioload before needing replacement.
Algae Reactors (Refugium-Style)
Algae reactors (sometimes called turf algae reactors or macro algae reactors) aren't bacteria-based but serve a similar purpose. They grow chaeto or turf algae in a dedicated chamber with targeted lighting. The algae absorbs nitrates and phosphates as nutrients for growth, and you export those nutrients by harvesting the algae.
The Pax Bellum ARID reactors and the Avast Marine Krusher are examples of dedicated algae reactor vessels. They're especially useful in reef tanks with high phosphate because algae excels at stripping phosphate compared to biopellets.
How to Set Up a Biopellet Reactor
Biopellet reactors are the most common type you'll encounter, so this section focuses on them.
Step 1: Size the Reactor
Most biopellet reactors list a maximum pellet volume they can tumble and a tank size they're designed for. The Two Little Fishies PhosBan Reactor 150 handles up to about 150ml of pellets and works for tanks up to 100 gallons. The Deltec FR509 is a larger format option for 200+ gallon systems.
A general starting dose is about 50ml to 100ml of pellets per 50 gallons of tank volume. Start at the lower end and increase slowly over several weeks.
Step 2: Place the Reactor and Plumb It
Most biopellet reactors use a small pump (like a Tunze 1073.008 or Cobalt Aquatics MJ1000) to circulate water through the reactor body. Water enters from the bottom, tumbles the pellets as it flows upward, and exits at the top. The reactor output should go directly to your protein skimmer inlet, not back into the sump first. The skimmer removes the bacterial biomass that the reactor produces, which is essential for the system to work.
Step 3: Ramp Up Slowly
Start with the flow rate low enough that pellets just barely tumble. As bacteria colonize the pellets over 2 to 4 weeks, you can increase flow. Full effectiveness usually takes 4 to 8 weeks from startup.
Test nitrates and phosphates weekly during startup. If either drops very fast in the first 2 weeks, reduce pellet volume slightly to avoid crashing levels that corals need for nutrition.
How Long Before You See Results?
Startup timelines vary based on tank maturity and existing bacterial populations.
In an established tank (6+ months old with a healthy biofilm throughout), a biopellet reactor typically shows measurable nitrate reduction within 2 to 3 weeks. Full effect by 6 to 8 weeks.
In a newer tank or one recovering from high nutrients, it takes longer because the bacterial population needs to establish. Sometimes 3 months before you see significant change.
The key indicator is the protein skimmer output. Once the biopellets are working, skimmer production increases noticeably. More skimmate means the bacterial turnover loop is functioning.
What Can Go Wrong
Oxygen depletion: Heavy bacterial growth consumes oxygen. If you're running a biopellet reactor in an enclosed sump with poor circulation, oxygen levels can drop, especially at night. Make sure your sump is well-aerated.
Overdosing pellets: More pellets aren't always better. Overloading a reactor causes pellets to cake and stop tumbling, which kills the bacteria and causes the pellets to break down too fast. Keep pellet volume within the manufacturer's recommended range for your reactor size.
Nuisance bacteria: Biopellets can encourage bacterial growth throughout the tank, not just in the reactor. If you see white filmy growth on rocks or glass, it may mean your pellet volume is too high relative to what the skimmer can process. Reduce pellets and increase skimmer output.
Skimmer running wet: A biopellet reactor connected to a skimmer often causes the skimmer to run wetter than before. This is normal and generally positive. If the skimmer cup overflows, raise the outlet pipe to concentrate skimmate more.
For equipment combinations that work well with bio reactors, including skimmers and circulation pumps, see our best aquarium equipment guide.
Bio Reactors vs. Other Nutrient Control Methods
| Method | Nitrate | Phosphate | Cost | Maintenance |
|---|---|---|---|---|
| Biopellet reactor | Excellent | Good | $80 to $200 setup | Monthly pellet check |
| Vodka/carbon dosing | Excellent | Good | $20 to $60/year | Daily dosing |
| Algae refugium | Good | Excellent | $100 to $300 setup | Weekly harvest |
| GFO reactor | None | Excellent | $50 to $150 setup | Monthly media change |
| Water changes | Good | Good | Variable | Weekly labor |
Most successful reef tanks combine two or three of these methods. Biopellets plus a GFO reactor is a common pairing that handles both nitrate and phosphate effectively. Biopellets plus chaeto in a refugium is another popular combination.
Our top aquarium equipment page has more detailed comparison information for nutrient export equipment.
FAQ
Do I need a bio reactor if I do regular water changes? It depends on your bioload and nutrient levels. Regular 10 to 15% weekly water changes are often sufficient for lightly stocked tanks with minimal feeding. For heavily stocked tanks or those with high target feeding for coral growth, a bio reactor helps maintain low nutrients between changes more effectively.
Can I run a biopellet reactor and a skimmer from the same pump? No. The biopellet reactor needs dedicated flow to tumble the pellets correctly. Running both from one pump usually provides inadequate flow to either. Use separate pumps, and route the reactor output to the skimmer inlet.
How do I know when my biopellets need replacing? Pellets shrink as they're consumed. When the pellet volume in the reactor has decreased by roughly 50%, it's time to add fresh pellets or replace the batch. Don't let the reactor run almost empty because the bacterial population crashes faster when the substrate runs out.
Can bio reactors work in freshwater tanks? Carbon dosing (vodka method) is occasionally used in heavily planted tanks to boost bacterial denitrification, but biopellet reactors are almost exclusively used in saltwater systems. Freshwater tanks have different nutrient cycles, and the bacteria that colonize biopellets are typically adapted to marine conditions.
Final Thoughts
A biopellet reactor is one of the more reliable tools for keeping nitrates and phosphates in check in a busy reef tank. Start conservatively with pellet volume, keep the output plumbed directly to your skimmer, and give it 6 to 8 weeks to reach full effectiveness before adjusting. Combined with a quality protein skimmer, it forms a strong nutrient export backbone for medium to large reef systems.