An aquarium calcium reactor is a device that dissolves calcium-rich media (typically aragonite or coral gravel) in a chamber of slightly acidified water, releasing calcium, magnesium, and alkalinity directly into your tank water. It uses CO2 to lower the pH inside the chamber, which dissolves the media and creates a calcium and alkalinity-rich effluent that drips continuously into the sump. For SPS-dominant reef tanks keeping fast-growing Acropora, Montipora, and other stony corals, a calcium reactor is often the most cost-effective and stable way to maintain calcium around 400 to 450 ppm and alkalinity around 8 to 10 dKH. For smaller tanks or soft coral setups, two-part dosing is simpler and equally effective.
This guide explains exactly how a calcium reactor works, when it makes economic and practical sense over other supplementation methods, how to size one for your tank, how to dial in the settings, and what problems to watch for. I'll also cover the CO2 equipment you'll need alongside the reactor itself.
How a Calcium Reactor Works
The calcium reactor operates as a closed acid dissolution system. Here's what actually happens inside the chamber.
The Dissolution Process
CO2 gas is injected into a sealed chamber partially filled with calcium carbonate media. The CO2 dissolves into the water inside the chamber, forming carbonic acid and lowering the chamber pH to roughly 6.5 to 6.8. At that pH level, calcium carbonate dissolves readily. The calcium-rich water continuously drips out of the reactor through an adjustable effluent valve and into the sump, where it mixes with the main tank water and raises calcium and alkalinity levels.
A recirculation pump inside the reactor keeps water and CO2 circulating together for better CO2 dissolution. Some reactors like the Reef Octopus MF-1000 use a separate secondary chamber to allow excess CO2 to off-gas before the effluent reaches the sump, which prevents pH swings in the main tank.
Why pH Matters in the Main Tank
The effluent from a calcium reactor has a pH of 6.5 to 7.0, which is acidic compared to your main tank's target of 8.1 to 8.3. If you're dripping effluent directly into a small sump, you need to ensure the effluent rate is slow enough that the main tank can buffer the acidity. A dual-chamber reactor, a bubble trap, or directing the effluent into high-flow areas of the sump prevents localized pH drops.
When a Calcium Reactor Makes Sense
The choice between a calcium reactor and two-part dosing or a kalkwasser reactor comes down to tank size, coral load, and ongoing cost.
Tank Size and Coral Load Thresholds
Two-part dosing (BRS Two-Part or ESV B-Ionic) is economical for tanks under 100 gallons with light to moderate coral growth. On a 180-gallon SPS-dominant tank, two-part consumption can run $60 to $120 per month. A calcium reactor uses $20 to $30 in CO2 refills and $15 to $30 in media per month, saving $30 to $80 monthly once the reactor is dialed in. The reactor hardware costs $250 to $700 depending on model, so the break-even point is typically 6 to 18 months.
Stability Advantage
A properly set calcium reactor drips effluent 24 hours a day, continuously replenishing what corals consume. Two-part dosing pumps dose in pulses, which creates small peaks and valleys in calcium and alkalinity levels. For most corals this difference is minor, but SPS corals, especially sensitive Acropora species, show better growth and less bleaching with steady calcium and alkalinity rather than fluctuating levels.
When Two-Part Is Better
For tanks under 75 gallons, for soft coral and LPS setups, or for aquarists who don't want the complexity of CO2 equipment, two-part dosing is simpler to manage and requires no additional equipment beyond a dosing pump. The cost difference at small scale doesn't justify the added setup.
For a full comparison of calcium supplementation equipment, the Best Calcium Reactor for Reef Tank guide compares the top models on the market.
Sizing a Calcium Reactor for Your Tank
Getting the right reactor size prevents undershooting your tank's calcium demand.
How to Estimate Your Calcium Demand
Calcium demand scales with coral mass and growth rate. A heavily stocked SPS tank consumes far more calcium than a FOWLR tank with a few soft corals. A practical rule: for a 100-gallon SPS tank, expect to need a reactor rated for at least 100 to 150 gallons. For a mixed reef with both LPS and SPS, size the reactor for your actual water volume.
Specific size recommendations: - 50 to 100 gallons, moderate reef: Two Little Fishies Kalkreactor 200 or the Aquamedic Carbo Calc 1.0 - 100 to 200 gallons, heavy SPS: Reef Octopus MF-800 or MF-1000, Deltec PF601 or PF1001 - 200+ gallons: Deltec PF-1001 or MTC Pro-Cal large-format reactors
Media Type Affects Dissolution Rate
ARM (Aragonite Reactor Media) is the most commonly used reactor media and dissolves efficiently at pH 6.5 to 6.8. Coral skeleton media dissolves more slowly and requires a lower pH. BRS High Purity Reactor Media and Two Little Fishies C-Balance media are popular choices. Top off the chamber before it drops below one-third full to maintain consistent effluent chemistry.
Setting Up a Calcium Reactor
Setup involves connecting the CO2 supply, calibrating effluent flow, and testing the effect on main tank parameters.
Required Equipment Beyond the Reactor
You'll need a CO2 cylinder, a CO2 regulator with solenoid and needle valve, and tubing to connect the CO2 source to the reactor. The same CO2 equipment used for planted tanks works, though calcium reactors typically use larger cylinders (5 to 20 pounds) since they run continuously. The Aquatek Mini CO2 Regulator or the Milwaukee MA957 are compact options for smaller setups. For more comprehensive CO2 regulator options, the Best CO2 Regulator for Calcium Reactor guide covers regulators matched to calcium reactor use.
Initial Calibration
- Fill the reactor chamber with media and prime with tank water.
- Connect CO2 and set a slow bubble rate into the reactor chamber, about 30 to 60 bubbles per minute.
- Set the effluent drip rate to roughly 20 to 40 drops per minute as a starting point.
- After 24 hours, test the effluent pH (should be 6.5 to 6.8) and the effluent calcium level (should be 400 to 600 ppm).
- Test main tank calcium and alkalinity daily for the first week. Adjust effluent rate up or down based on whether tank parameters are rising, falling, or stable.
The goal is an effluent drip rate that replaces what corals consume without causing large swings. Most tanks stabilize within 1 to 2 weeks of adjustment.
CO2 Consumption and Ongoing Costs
A calcium reactor uses CO2 continuously, but consumption is low compared to planted tank injection rates.
Cylinder Life
A 10-pound CO2 cylinder lasts 3 to 6 months on a calcium reactor for a 100-gallon tank with moderate coral load. At around $20 to $30 per fill, annual CO2 cost is $40 to $80. A 20-pound cylinder lasts proportionally longer. This is the main recurring consumable beyond media.
Media Consumption
Media consumption depends on your tank's calcium demand. Expect to top up a full reactor chamber every 2 to 4 months on a 100-gallon reef. A 10-pound bag of BRS ARM media retails around $20 to $25 and typically fills a mid-size reactor chamber once. Annual media cost is $60 to $150 depending on tank size and coral load.
Common Problems and Solutions
Effluent pH Too High (Above 7.0)
This means CO2 isn't fully dissolving in the reactor chamber. Increase CO2 bubble rate slowly, check that the recirculation pump is working, and verify that CO2 is actually entering the chamber and not escaping at a connection point. Also check the reactor's bubble counter and needle valve for partial blockage.
Effluent pH Too Low (Below 6.2)
This causes excessive dissolution and can deplete media quickly. Reduce CO2 rate or increase effluent flow to reduce contact time. Effluent pH below 6.0 can introduce enough acidity to affect main tank pH.
Calcium Rises But Alkalinity Doesn't (or Vice Versa)
Calcium and alkalinity should rise together since both come from dissolving calcium carbonate. If one rises disproportionately, check whether your test kits are accurate. Calibrate with a reference solution or send a sample to ICP testing. If one parameter is genuinely out of balance, adjust by supplementing the lagging parameter with two-part solution until the reactor stabilizes.
Media Dissolves Too Fast
Using very fine media or pure aragonite sand causes faster dissolution than intended. Switch to larger-particle ARM media or calcium reactor-grade crushed coral, which dissolves more predictably. Mixing media types is also common: 80% ARM and 20% coral skeleton media slows dissolution and increases buffering capacity.
FAQ
Do I need a calcium reactor for a reef tank? Not necessarily. For tanks under 75 gallons or for setups dominated by soft corals and LPS, two-part dosing is simpler and costs about the same. Calcium reactors make the most sense for tanks 100 gallons and larger with significant SPS coral growth, where the monthly savings on dosing chemicals offset the equipment cost within a year.
What media should I use in my calcium reactor? BRS ARM (Aragonite Reactor Media) is the most widely used and recommended starting media. It dissolves consistently at a pH of 6.5 to 6.8 and is widely available. Two Little Fishies C-Balance media and Deltec's own reactor media are also popular. Avoid very fine substrate like aragonite sand, which dissolves too quickly and compacts.
Can I run a calcium reactor on a 30-gallon reef tank? You can, but it's generally not worth the complexity or cost at that scale. A 30-gallon mixed reef with a few LPS corals can be maintained easily with manual two-part dosing or even daily Kalkwasser additions. The smallest practical calcium reactor size (units like the Aquamedic Carbo Calc 1.0) is really designed for tanks 50 gallons and up.
How long does it take to dial in a calcium reactor? Expect 1 to 3 weeks of daily testing and small adjustments before the reactor is producing the right effluent and maintaining stable main tank parameters. Once dialed in, most calcium reactors run for months without needing adjustment, assuming coral growth rate stays fairly constant. A sudden rapid growth phase after dosing coral nutrients may require a slight increase in effluent rate.