A calcium reactor is a piece of equipment that dissolves calcium carbonate media using CO2-acidified water to continuously replenish calcium, alkalinity (carbonate hardness/KH), and magnesium in a reef tank. Instead of dosing two-part solutions every day or relying on a kalkwasser drip, a properly tuned calcium reactor handles the bulk of your coral's mineral demand automatically. For tanks with high stony coral (SPS) loads, it's one of the most reliable and cost-effective supplementation methods available.
This guide walks through how calcium reactors work, how to size one for your system, how to set one up and dial it in, what media to use, and how to troubleshoot the most common problems. You'll also find a frank comparison of calcium reactors versus two-part dosing so you can decide which approach makes sense for your tank right now.
How a Calcium Reactor Works
The core process happens inside a sealed reaction chamber. A small recirculating pump draws tank water into the chamber and passes it over a bed of calcium carbonate media (usually crushed aragonite, coral skeleton, or dolomite). CO2 is bubbled into the chamber simultaneously, where it dissolves into the water to form carbonic acid. That mild acid dissolves the calcium carbonate media, releasing calcium ions, carbonate alkalinity, and trace elements back into the water. The effluent (the output of the reactor) then drips back into the sump at a controlled rate, replenishing what corals consume.
Key Components
Every calcium reactor system includes:
- The reactor chamber itself, which holds the media and houses the recirculating pump
- A CO2 cylinder and regulator specifically rated for aquarium use. A dual-stage regulator is strongly recommended because single-stage regulators can "end of tank dump" when the cylinder gets low, flooding the reactor with CO2 and crashing pH. Check our Best CO2 Regulator for Calcium Reactor guide for vetted options.
- A bubble counter to monitor CO2 input
- A check valve between the regulator and reactor to prevent water backflow
- A pH controller or pH meter to monitor the effluent pH inside the chamber
- A needle valve or dosing pump to control the effluent drip rate back to the sump
The pH inside the reactor chamber typically runs between 6.5 and 6.8. At this pH, the media dissolves steadily without dissolving too fast or stalling.
Sizing a Calcium Reactor for Your Tank
Calcium reactors are sized based on coral demand, not just tank volume. An SPS-dominant 120-gallon system consumes far more calcium than a 120-gallon FOWLR (fish only with live rock) tank.
A rough sizing guide based on calcium consumption:
| Coral Load | Tank Size | Recommended Reactor Size |
|---|---|---|
| Light (LPS, softies) | Up to 150 gallons | Korallin C-1502, Aquamaxx CaRx 1.5 |
| Moderate (mixed reef) | 75-200 gallons | Two Little Fishies Knop S-IV, Reef Octopus CR-140 |
| Heavy (SPS dominant) | 100-300+ gallons | Korallin C-3001, Reef Octopus CR-200, ELOS CM120 |
The Korallin C-1502 handles moderate mixed reefs well at around $180 to $220. For SPS-heavy systems, the Reef Octopus CR-200 is popular, with a larger reaction chamber and reliable recirculating pump, typically priced around $300 to $400. Our Best Calcium Reactor for Reef Tank roundup has side-by-side comparisons if you want to dig deeper.
Estimating Your Calcium Demand
Stony corals consume roughly 400 mg of calcium and 2.8 to 3.0 meq/L of alkalinity per liter of tank volume per week in a heavily stocked system. A 100-gallon SPS tank might consume 2,000 to 3,000 mg of calcium weekly. Your calcium reactor needs to produce enough effluent to replace this continuously. As a starting point, most hobbyists run effluent at 20 to 60 drops per minute and adjust from there based on water testing.
Choosing Your Calcium Reactor Media
The media you load into the reactor determines how quickly it dissolves, how long it lasts, and what trace elements it contributes.
ARM (Aragonite Reactor Media)
CaribSea ARM (Aragonite Reactor Media) is the most widely used option. It's a coarse crushed aragonite that dissolves at a predictable rate, provides good calcium and alkalinity output, and contributes trace magnesium. Available in 10-pound bags for around $20 to $25. It works in virtually every reactor.
Dolomite
Dolomite media (like Two Little Fishies CalcMag) contains both calcium carbonate and magnesium carbonate. It dissolves more slowly than pure aragonite, which means it lasts longer and is well-suited as a secondary media layer. Running 25 to 30% dolomite mixed with aragonite helps maintain magnesium levels without separate dosing.
Coral Rubble
Some hobbyists use aquacultured coral rubble as media. It's effective and sustainable, but results vary by source because purity and composition differ. It's not standardized the way commercial reactor media is, so I'd stick with ARM or Korallith for a dialed-in system.
Setting Up and Dialing In a Calcium Reactor
Setup involves more steps than most aquarium equipment, but once it's running, adjustments are minor tweaks.
Installation Steps
- Place the reactor in or next to your sump. Most reactors sit in a sump section or on a shelf next to the tank stand.
- Connect the recirculating pump inlet to the sump. Water from the sump feeds the reactor.
- Set up the CO2 line with a check valve and bubble counter between the regulator and the reactor's CO2 inlet.
- Fill the chamber with media. Load the reactor body, cap it, and manually fill it with RO/DI water to purge air.
- Set the effluent output to drip back into the sump. Route the effluent line to a stable section of sump water.
- Start the CO2 at 1 bubble every 2 to 3 seconds. This is deliberately conservative for startup.
- Check the effluent pH using a pH meter or controller. The target is 6.5 to 6.8 inside the chamber.
- Test tank calcium and alkalinity daily for the first two weeks and adjust the effluent drip rate up or down to match consumption.
Understanding the Two Adjustment Points
You have two variables to tune: CO2 bubble rate and effluent drip rate. CO2 controls how acidic the water inside the reactor gets, which controls how fast media dissolves. The drip rate controls how much of that dissolved calcium/alkalinity returns to the tank per hour.
In practice: increase CO2 if your effluent pH is above 6.8 (media not dissolving enough). Reduce CO2 if pH drops below 6.5 (over-acidified, which can also push too much CO2 back to the tank and depress main tank pH). Increase the drip rate if tank calcium or alkalinity is trending down. Decrease it if parameters are drifting too high.
Calcium Reactor vs. Two-Part Dosing
This is the question every reef hobbyist faces before committing to either approach.
Two-part dosing (brands like BRS Two-Part, Bulk Reef Supply, or ESV) is simpler to start. You buy calcium chloride and sodium bicarbonate solutions, set a dosing pump, and the system runs. Startup cost is around $100 to $150 for a dosing pump and initial chemicals. But ongoing costs add up, especially for high-demand SPS tanks. A heavily stocked 100-gallon SPS system can easily spend $40 to $80 per month on two-part solutions.
Calcium reactor has higher upfront cost ($200 to $400 for reactor plus CO2 equipment) but ongoing costs drop to roughly $5 to $10 per month for CO2 and $20 per year for media. For tanks consuming significant amounts of calcium, a reactor typically breaks even within 12 to 18 months and runs cheaper thereafter indefinitely.
The other consideration is stability. A calcium reactor that's properly dialed in delivers calcium and alkalinity continuously and proportionally to coral demand. Two-part dosing in small bolus doses can create parameter swings if the dosing schedule doesn't match exactly what corals consume each day.
For a mixed reef or a modest SPS setup, two-part is fine and simpler. For an SPS-dominant tank or any system with more than 50 to 75 frags or colonies, a calcium reactor starts making both practical and financial sense.
Troubleshooting Common Problems
Tank pH is dropping. If your main tank pH is sliding down after adding the calcium reactor, the effluent may be too acidic, or you're running too high a drip rate. Check the effluent pH and reduce CO2 input until chamber pH is at 6.7 to 6.8. Also confirm the reactor effluent is dripping into well-aerated sump water, not directly into stagnant areas.
Calcium and alkalinity are still declining despite the reactor running. The reactor may be undersized for your coral load, or the media bed may be depleted. Check the media level (it should fill at least half the chamber), and increase the effluent drip rate gradually while retesting.
CO2 is running out very quickly. A typical 5-pound CO2 cylinder lasts 3 to 6 months on a medium-sized reef. If you're going through it in weeks, check for leaks at every connection using soapy water. A slow leak is easy to miss but expensive over time.
Effluent pH won't drop below 7.0 no matter how much CO2 you add. Usually means there's too much flow through the reactor relative to the CO2 input, or the recirculating pump is underpowered. Reduce the feed pump flow rate or upgrade the recirculating pump.
FAQ
Do I need a calcium reactor if I already dose two-part? You don't need both simultaneously. A calcium reactor replaces two-part dosing for calcium and alkalinity supplementation. Some hobbyists run both during the transition period while dialing in the reactor, but operating both long-term is redundant and can cause parameter instability.
What's a safe effluent drip rate to start with? Start at 20 drops per minute (roughly 1 to 1.5 liters per day), test calcium and alkalinity every 2 to 3 days for the first two weeks, and increase or decrease from there. Changes take a day or two to show in tank parameters, so don't make big adjustments every day.
How often do I need to add media? Depends on reactor size and coral demand. On a moderately loaded 100-gallon reef, a 10-pound bag of ARM media might last 2 to 4 months. Check the media level monthly and top off before it gets below one-third of the chamber capacity. A fully depleted chamber causes the effluent pH to rise and output to drop off sharply.
Can I run a calcium reactor on a small tank? Technically yes, but it's often not cost-effective. A calcium reactor and CO2 system costs $200 to $400 to set up. For tanks under 50 gallons with moderate coral loads, two-part dosing is almost always simpler and cheaper. Calcium reactors really earn their place on tanks 75 gallons and larger with meaningful SPS populations.
Conclusion
A calcium reactor is one of the best investments you can make for a mature SPS reef. The upfront cost pays off within a year or two on any system with serious coral demand, and the stability of continuous supplementation beats bolus dosing once the reactor is properly dialed in. Focus on a quality dual-stage CO2 regulator, use ARM or a mixed ARM/dolomite media bed, and plan on spending the first two weeks making small adjustments while your tank parameters stabilize. After that, maintenance is minimal and your corals respond noticeably.