Saltwater Tank Reactor

A saltwater tank reactor is a sealed chamber that passes water through a specific media or substance to achieve a targeted chemical effect. Reef tanks use reactors to control calcium, alkalinity, phosphate, nitrate, and organic compounds, each with a different reactor type designed for the job. Most reef keepers start with a protein skimmer and progress to adding reactors as the tank matures and water chemistry demands become more precise.

This guide covers all the major reactor types used in saltwater tanks, how each one works, which are worth buying, and how to build a reactor system that fits your tank size and livestock.

Calcium Reactors: The Cornerstone of SPS Reef Keeping

The calcium reactor is the most talked-about reactor in saltwater keeping, and it earns that attention. Stony corals (especially SPS like Acropora, Montipora, and Stylophora) extract calcium and carbonate from the water to build their skeletons. A tank with significant SPS coverage can deplete alkalinity by 1-2 dKH per day. A calcium reactor replenishes those elements continuously without daily dosing.

How It Works

The reactor chamber is filled with calcium carbonate media, typically crushed aragonite, ARM Coarse, or BRS Reef Saver Rock. CO2 is injected into the chamber, lowering internal pH to around 6.5-6.7. At that pH, the calcium carbonate media dissolves, releasing calcium and carbonate into the water flowing through the chamber. The effluent drips back into the sump, maintaining stable calcium and alkalinity without manual dosing.

You need three components: the reactor body, a CO2 cylinder (5-pound cylinders are standard for home use), and a CO2 regulator. Milwaukee MA957 and Aqua Medic 3000 Twin regulators are reliable entry-level options at $50-80. The reactor itself ranges from $150 for a basic Two Little Fishies C-1 to $400+ for a BRS Dual Chamber Reactor for larger systems.

When You Need One

Calcium reactors make financial sense when your two-part dosing costs exceed roughly $30-50 per month. For a small mixed reef under 75 gallons with moderate coral coverage, two-part is often simpler. For a 150+ gallon SPS-dominant system, a calcium reactor pays for itself within a few months.

Phosphate Reactors: Controlling Nuisance Algae

Elevated phosphate is one of the top causes of algae outbreaks and stunted coral growth in reef tanks. The target for a mixed reef is 0.03-0.05 ppm; SPS tanks often run 0.02-0.03 ppm. Above 0.1 ppm, hair algae accelerates, corals brown out, and coralline algae struggles.

Phosphate reactors use granular ferric oxide (GFO) media. As water passes through the chamber, iron in the GFO adsorbs phosphate. The Two Little Fishies PhosBan Reactor 150 is the most common entry-level unit and works well for tanks up to about 150 gallons. The BRS Single Reactor uses a recirculating design that's slightly more efficient. For larger systems, the Deltec PF501 and Aqua Medic Phosphate Reactor 400 handle higher flow rates.

GFO Dosing Protocol

Start with less media than recommended, especially in an established tank with elevated phosphate. Dropping phosphate rapidly from 0.3 ppm to near zero stresses corals that have acclimated to higher levels. Use half the recommended media for the first two weeks, test twice weekly, and increase media volume slowly as corals show no stress response.

BRS High Capacity GFO is a popular choice for its coarser granule size, which reduces turbidity if flow rate fluctuates. Replace media every 4-8 weeks or when phosphate begins rising again.

Carbon Reactors: Water Clarity and Organic Control

A carbon reactor forces water through activated carbon media under controlled flow, providing much more contact time than passive carbon in a bag. This matters when you're dealing with persistent yellowed water, dissolved organics following a die-off, or post-medication cleanup.

The same reactor body (Two Little Fishies PhosBan 150 or BRS Single Reactor) can run activated carbon with a different pump speed setting. BRS ROX 0.8 carbon is preferred for reef use because it's acid-washed and won't leach phosphate, which cheaper carbons sometimes do.

Most established reef tanks with a properly tuned protein skimmer don't need continuous carbon. Running it for 2-3 weeks following a tank disturbance, then removing it, is a common protocol. Long-term continuous carbon can strip trace elements.

Biopellet Reactors: Addressing Nitrate and Phosphate Together

Biopellet reactors use solid carbon-source polymer pellets (typically PHB or PHA polymers) to feed bacteria that simultaneously consume nitrate and phosphate. The bacteria grow on the pellets inside the tumbling reactor chamber, consume dissolved nitrate and phosphate, and are then exported from the system via the protein skimmer.

Two Little Fishies NPX BioPellets and BRS Biopellets are the standard media options. Biopellet reactors work best in heavily stocked systems where nutrients are persistently high despite good skimming. They require a properly sized skimmer to export the bacterial load they generate.

Important: bacterial blooms in the first 2-4 weeks can temporarily cloud the water and stress corals. Start with 1/3 of the recommended pellet volume and ramp up over 4-6 weeks.

Kalkwasser Reactors (Kalk Stirrers): Simple Calcium and Alkalinity Management

A kalk stirrer or Kalkwasser reactor is a simpler reactor concept: it dissolves calcium hydroxide (kalkwasser) into fresh water, which is then used for evaporation top-off. As the tank loses water to evaporation, fresh kalk-saturated water replaces it, simultaneously replenishing calcium and alkalinity and maintaining salinity.

Aqua Medic Kalkwasser Stirrer and Two Little Fishies Kalkwasser Stirrer are commonly used. These work well for small to medium reefs (under 150 gallons) as an alternative to two-part dosing and as a step before a full calcium reactor. Kalk is cheaper than two-part and covers evaporation-based top-off automatically.

Limitation: Kalkwasser has a fixed calcium-to-alkalinity ratio. If your tank consumes them at different rates (which SPS-heavy systems often do), you'll need supplemental adjustments.

Building a Complete Reactor System

For a complete reference on how reactors fit into a full saltwater filtration setup, check our best aquarium equipment guide, which covers skimmers, reactors, lighting, and return pumps together. Our top aquarium equipment roundup includes specific model comparisons and pricing.

A sensible progression for most saltwater tanks looks like this:

Stage 1 (New tank, 0-12 months): Protein skimmer + two-part dosing or Kalkwasser. No reactor needed yet.

Stage 2 (Established tank, growing SPS): Add a calcium reactor when two-part costs justify it, or when calcium and alkalinity are hard to keep stable.

Stage 3 (High nutrient, heavy stocking): Add a phosphate reactor if phosphate climbs above 0.1 ppm despite water changes and good skimming.

Stage 4 (Optimization): A carbon reactor for periodic use, biopellets if nitrate is persistently elevated.

Frequently Asked Questions

Can I run a calcium reactor without a CO2 controller? You can, but a pH controller makes it much easier to maintain stable effluent. Without one, you set a fixed CO2 bubble count and manually adjust based on tank alkalinity test results. A Milwaukee MC122 or Apex DOS pH controller automates this by throttling CO2 based on the measured chamber pH.

Do I need a reactor if I'm only keeping soft corals and fish? No. Soft corals have low calcium demands and tolerate the ranges maintained by regular water changes and Instant Ocean Reef Crystals salt mixes. Reactors become necessary when stony coral coverage creates calcium and alkalinity demand that exceeds what simple maintenance can supply.

What's the difference between a calcium reactor and kalkwasser for a 50-gallon reef? For a 50-gallon tank with moderate LPS coverage, kalkwasser is usually simpler and cheaper. A kalk stirrer costs $50-100 and works passively with your ATO system. A calcium reactor costs $200-400 plus CO2 equipment and requires ongoing dialing-in. The reactor makes more sense as coral coverage increases and kalk can't keep pace with demand.

How do I test if my calcium reactor is working correctly? Measure tank calcium (target: 400-430 ppm for most reefs) and alkalinity (target: 8-10 dKH) twice per week when first running the reactor. If both values are stable over two weeks, the reactor is dialed in. If calcium is rising but alkalinity is dropping (or vice versa), adjust effluent drip rate and check CO2 level in the chamber.

Reactors for saltwater tanks are precision tools that become valuable at specific stages of reef keeping. A calcium reactor solves the calcium and alkalinity grind in mature SPS systems. A phosphate reactor addresses persistent nutrient issues directly. Understanding what each type does and when your tank actually needs it prevents unnecessary spending and keeps your reef stable.