Reactors for reef tanks come in several types, each designed to control a specific aspect of water chemistry. Calcium reactors stabilize calcium and alkalinity for coral growth. Phosphate reactors remove excess phosphate using iron-based media. Carbon reactors improve water clarity by forcing contact with activated carbon. Each solves a real problem, but they're not all equally necessary, and adding the wrong one at the wrong time can create more issues than it solves.
This guide covers all the major reactor types you'll encounter in reef keeping, how they work, what they cost, which brands are worth buying, and how to decide which one your tank actually needs. I'll also explain the order in which most successful reefers add this equipment.
Calcium Reactors: Keeping SPS Reefs Stable
Calcium reactors are the most commonly discussed reactor type in reef keeping, and for good reason. In a tank with significant stony coral coverage, calcium and alkalinity get consumed fast. A single large Acropora colony can consume measurable amounts of calcium daily, and a reef tank with 30-40% SPS coverage can deplete alkalinity by 0.5-1.0 dKH per day.
Two-part dosing solutions (BRS Two Part, Randy Holmes-Farley DIY two-part) work well for small or lightly stocked systems. But as coral coverage increases, the cost and volume of two-part dosing increases proportionally. At some point, typically around the equivalent of one gallon of part A per week, a calcium reactor becomes more economical.
How Calcium Reactors Function
The reactor chamber holds calcium carbonate media. CO2 is injected to lower internal pH to approximately 6.5-6.8, which causes the media to dissolve and release calcium and carbonate ions into the effluent water that drips back into the sump.
Critical parameters to dial in: - Effluent pH: Measure with a pH probe. Target 6.5-6.8 in the reactor. - Effluent drip rate: Start slow (30-60 drops per minute) and adjust based on whether system calcium and dKH are rising or falling - CO2 bubble count: Match to effluent pH rather than a fixed number
Calcium Reactor Recommendations
The Two Little Fishies C-1 handles tanks up to about 75 gallons with moderate coral loads. The BRS Dual Chamber Calcium Reactor is a reliable mid-range choice for 100-200 gallon systems. For serious SPS tanks above 200 gallons, the GEO 612, Korallin C-1502, and Aqua Medic 2000 are all proven. Budget $200-400 for the reactor body, plus $50-100 for a CO2 regulator (Milwaukee MA957 or Aqua Medic 3000 Twin).
Reactor media choices: ARM Coarse is the most popular for its dissolution rate. BRS Reef Saver Rock media dissolves more slowly, giving more stable effluent. Hawaiian Black provides calcium and magnesium together.
Phosphate Reactors: Tackling Nutrient Problems
Elevated phosphate is one of the most common causes of nuisance algae and stunted coral growth in reef tanks. The target range for a mixed reef is 0.03-0.05 ppm; SPS-dominant tanks often target 0.02-0.03 ppm. When phosphate climbs above 0.1 ppm, you'll see hair algae acceleration, cyano outbreaks, and visible brown coloration in SPS corals.
Phosphate reactors use granular ferric oxide (GFO) as media. Iron in the GFO binds to phosphate molecules as water flows through the chamber. Unlike chemical precipitation in a sump, a reactor maintains consistent contact between water and media, making it much more efficient than passive placement.
Running GFO Safely
This is where new reef keepers make mistakes. GFO works fast, and dropping phosphate rapidly stresses corals that have acclimated to elevated levels. If you're starting at 0.3 ppm phosphate, don't fill the reactor and run it at full capacity. Add half the recommended media and run the reactor at reduced flow for two weeks, check levels, and increase gradually.
Recommended media: BRS High Capacity GFO, Two Little Fishies PhosNet, and Salifert PhosEx are all reliable. BRS High Capacity is coarser and produces less turbidity if flow rate spikes.
Dual-Purpose Reactor Bodies
The Two Little Fishies PhosBan Reactor 150 runs either GFO or activated carbon and is the most popular reactor body in the hobby for this purpose. The BRS Single Reactor uses a recirculating pump design that's more efficient for GFO. For larger systems, the Deltec PF501 and Pacific Sun Phosban 200 handle higher flow rates.
Carbon Reactors: Water Clarity and Organic Control
A carbon reactor forces water through activated carbon media for maximum contact time. Passive carbon in a mesh bag in the sump sees poor water flow and requires replacement more often. A reactor with a small pump like the MJ900 or Rio 90 runs water through the carbon bed continuously, extending media life and improving effectiveness.
This matters most in tanks with: - Dense fish populations that generate heavy dissolved organic loads - Recent disease treatment with medications that need to be removed post-treatment - Persistent water yellowing even with a well-tuned protein skimmer - Post-algae-bloom cleanup when dissolved organics spike
ROX 0.8 activated carbon (from BRS) is highly regarded for reef use because it's acid-washed and won't leach phosphate, which many cheaper carbons do. Budget carbon can actually worsen phosphate problems in reef tanks, so source quality matters.
Carbon doesn't need to run constantly in most established reef tanks. Running it for 2-3 weeks after a system disturbance, then removing it, is a common approach. Continuous carbon use can strip some trace elements that corals use.
Biopellet Reactors: Nitrate and Phosphate Together
Biopellet reactors use solid carbon source pellets (usually PHA or PHB polymers) to feed bacteria that consume nitrate and phosphate simultaneously. As bacteria grow on the pellets, they export nitrate and phosphate from the water column. The bacterial biomass is then removed by the protein skimmer.
NPX Bioplastics biopellets (distributed by Two Little Fishies) and BRS Biopellets are the most common media. The tumbling action inside the reactor (created by the pump) keeps pellets moving and prevents channeling.
Biopellet reactors work best with a skimmer able to handle the increased bacterial load. They're popular in nutrient-heavy systems with heavy fish stocking. One caution: bacterial blooms in the first few weeks can cloud the water and temporarily stress corals. Start with less than the recommended dose and ramp up over a month.
Which Reactor Does Your Tank Need First?
The answer depends on your tank's age and what you're keeping.
New tank under 6 months: No reactor needed. Focus on skimmer performance, water change consistency, and stable parameters with two-part or Kalkwasser dosing.
Established mixed reef with moderate SPS: A calcium reactor starts making economic sense at this stage, especially if you're spending $50+/month on two-part solution.
Any tank with persistent phosphate above 0.1 ppm: A phosphate reactor with GFO is your most direct solution. Address the source (feeding, bioload) at the same time.
Tank with water clarity issues: A carbon reactor run periodically solves water yellowing efficiently.
Check out our best aquarium equipment guide for full product comparisons, and our top aquarium equipment roundup covers specific reactor models with pricing and user feedback.
Frequently Asked Questions
Can I run a calcium reactor and two-part dosing at the same time? Yes, and some reefers do this during the reactor break-in period. Once the reactor is dialed in and maintaining stable calcium and alkalinity, two-part dosing becomes redundant and can be discontinued. Running both long-term wastes money and can cause swings if both are adding calcium and alkalinity simultaneously.
How do I know if my phosphate reactor GFO is exhausted? Test phosphate weekly. When phosphate starts rising again after a period of being stable, the GFO is saturated. You can also test by removing the reactor briefly: if phosphate spikes within a few days, the GFO was still working. Replace media every 4-8 weeks in a reef with moderate phosphate load.
Do biopellet reactors replace the need for a protein skimmer? No. Biopellet reactors require a protein skimmer to work. The bacteria grown on the pellets need to be exported from the system, and the skimmer does that job. Without a skimmer, the bacterial biomass dies and rots back into the water, worsening nitrate and phosphate rather than helping.
What size CO2 tank do I need for a calcium reactor? For most home reef tanks, a 5-pound CO2 cylinder lasts 6-12 months. A 10-pound tank cuts refill trips in half. CO2 tanks can be refilled at welding supply stores, homebrew shops, and some paintball retailers. The Milwaukee MA957 regulator and comparable Aqua Medic units work with standard CGA-320 valve cylinders.
Reactors for reef tanks are purpose-built tools for specific water chemistry challenges. A calcium reactor earns its place in a mature SPS tank where two-part costs mount up. A phosphate reactor addresses nutrient issues directly when other methods fall short. Carbon reactors give you a targeted tool for organic control. Add them as your tank demonstrates the need, and you'll spend less and get better results than buying everything upfront.