A calcium reactor dissolves calcium carbonate media using CO2-acidified water to release calcium and alkalinity directly into your reef tank. In plain terms, it's a self-replenishing dosing system that replaces the two-part solutions most hobbyists start with, and it does the job continuously without you manually measuring and pouring anything every day.
If you're running a lightly stocked mixed reef up to about 100 gallons, you might not need one yet. But once you have several large SPS colonies or a heavily loaded LPS tank, a calcium reactor often becomes the most cost-effective and stable method for keeping calcium around 420 ppm and alkalinity around 8.5 dKH without constant intervention.
How a Calcium Reactor Actually Works
The core concept is simple: water from your sump gets pumped into a sealed chamber filled with calcium carbonate media (usually crushed aragonite or coral skeleton). CO2 is injected into that chamber, which lowers the pH inside the reactor to around 6.5. At that lower pH, the media dissolves, releasing calcium and bicarbonate ions. The effluent, now rich in both, drips back into your sump.
The CO2 Circuit
A CO2 cylinder connects to a regulator, which controls pressure and bubble count. The regulator feeds a bubble counter (so you can actually see your bubble rate) and then routes gas into the reactor chamber. Most reefers run somewhere between 20 and 60 bubbles per minute depending on their tank's demand.
The effluent pH and drip rate are your two main adjustment handles. Typical target effluent pH is 6.5 to 6.8. Too low and you dissolve media too aggressively, producing unstable, low-pH water. Too high and the media barely dissolves, giving you minimal output.
Recirculation Pump
Inside the reactor, a small recirculation pump keeps water moving through the media bed. This prevents channeling (where water carves a path around media rather than through it) and maximizes dissolving efficiency. Brands like Two Little Fishies (the PhosBan Reactor 550 is sometimes repurposed, but purpose-built calcium reactors like the Reef Octopus or Korallin C-1502 include dedicated recirculation pumps).
Choosing the Right Calcium Reactor Media
Reactor media is not all the same. Your two main options are aragonite and calcite, and they behave differently.
Aragonite (like Caribsea ARM Coarse or Two Little Fishies Reborn) dissolves more readily at higher pH, which means you don't need to push your effluent pH as low. It produces a more complete mineral profile because it more closely matches natural reef substrate.
Calcite is denser and dissolves more slowly. Some reefers prefer it because it lasts longer between refills, but you typically need lower effluent pH to get the same output.
Most reefers use ARM Coarse as a starter media. A 15-lb bag runs around $30 to $40 and lasts six months to a year in a moderately stocked 100-gallon reef. You'll add a top-off dose when the chamber gets about one-third empty rather than waiting for it to run out completely.
Magnesium and Secondary Minerals
One benefit of quality aragonite media is that it releases trace amounts of magnesium, strontium, and other minor elements alongside calcium and alkalinity. This isn't enough to replace dedicated magnesium dosing in a heavy SPS tank, but it does reduce how often you need to supplement.
Sizing a Calcium Reactor for Your Tank
Match reactor size to your tank's calcium and alkalinity demand. Manufacturers usually list rated tank volume, but those numbers assume moderate coral load. I'd suggest sizing up by 20 to 30 percent if you're running a full SPS display.
Common pairings: - 20 to 75 gallon reef: Korallin C-1501 or Two Little Fishies C-1502 - 75 to 150 gallon reef: Korallin C-1502, Reef Octopus CR-100, or Geo 612 - 150 to 300 gallon reef: Geo 618, Reef Octopus CR-150, or a dual-chamber unit like the Knop C
Dual-chamber reactors add a second chamber downstream of the first. The second chamber brings effluent pH back up slightly by passing it through fresh media, which reduces the impact of low-pH water dripping into your sump. This is worth considering in larger tanks where effluent volume becomes significant.
For specific model comparisons, the Best Calcium Reactor for Reef Tank guide breaks down current options by tank size and budget.
Setting Up and Dialing In Your Reactor
Setup takes an afternoon. The basic sequence is: install the reactor in your sump, connect the CO2 line from regulator to reactor, set up the feed pump pulling water from the sump into the reactor, and run the effluent drip line back into the sump.
Initial Startup
Fill the chamber with media, add water, then slowly introduce CO2. Watch your bubble counter and start at 20 bubbles per minute. Let the reactor run for 24 hours before adjusting anything. Check effluent pH with a calibrated pH probe or controller and adjust bubble rate from there.
Matching Output to Demand
Use an alkalinity test kit (Salifert or Red Sea Pro) to check your tank's dKH every day for the first two weeks. You want to see a stable number, not one that drifts up or down by more than 0.5 dKH per day. If alkalinity is rising, lower your effluent drip rate or reduce bubble count. If it's falling, increase one or both.
A Milwaukee pH controller connected to your CO2 solenoid can automate this: set it to open the solenoid when pH in the reactor drops below 6.7 and close it above 6.5. This keeps effluent pH in range without manual tuning.
CO2 Regulator Selection
The regulator is one place you don't want to cut corners. An end-of-tank dump, where a failing regulator suddenly floods your reactor (and tank) with CO2, can crash pH and kill livestock. Choose a regulator designed for aquarium use with a needle valve for fine adjustment and a reliable solenoid.
Popular options include the Milwaukee MA957 (budget-friendly, works fine for smaller reactors), the Aquatek CO2 Regulator Mini, and the Tunze 7077/3 for those who want precise electronic control. Pair whichever regulator you choose with a quality bubble counter and check it daily until the system is dialed in.
The Best CO2 Regulator for Calcium Reactor article covers the top-rated regulators with side-by-side comparisons on reliability and price.
Calcium Reactor vs. Two-Part Dosing
Both methods work. Here's the practical difference:
Two-part dosing (like BRS 2-Part or Randy Holmes-Farley DIY) doses calcium chloride and sodium bicarbonate solutions separately. It's simple to start, precise with a dosing pump, and costs around $30 to $50 per month for a heavily stocked 100-gallon reef. You need to refill bottles every few weeks and recalculate dose as coral demand grows.
Calcium reactor has higher upfront cost ($200 to $600+ for reactor plus CO2 equipment) but ongoing cost is very low, mainly just CO2 refills (around $5 to $15 for a 5-lb cylinder exchange) and media. For tanks consuming over 100 grams of calcium per week, the reactor typically pays for itself within a year compared to two-part.
The stability advantage of a calcium reactor is real. Because it doses continuously rather than in timed pulses, your parameters don't spike and crash between doses. For SPS-dominant tanks where even a 0.5 dKH swing causes bleaching, that consistency matters.
FAQ
Do I need a pH controller with a calcium reactor? Not required, but strongly recommended. A pH controller like the Milwaukee MC122 automates CO2 delivery and prevents end-of-tank dumps. Budget around $50 to $80 for a basic controller with probe.
How long does calcium reactor media last? A 15-lb bag of ARM Coarse typically lasts 6 to 12 months in a 100-gallon reef with moderate SPS. You'll see the media level drop gradually; top up when it reaches the lower third of the chamber.
Will a calcium reactor lower my tank pH? Yes, slightly. The effluent comes out at low pH, and even after mixing with your sump water, it can pull display tank pH down by 0.05 to 0.1 units. Using a dedicated effluent chamber or injecting into high-flow areas of the sump minimizes this.
Can I use a calcium reactor on a FOWLR tank? You can, but it's usually overkill. Fish-only tanks with live rock don't consume significant calcium or alkalinity, so two-part or simple kalkwasser top-off is more practical.
Conclusion
A calcium reactor makes the most sense once your reef tank has enough coral demand that two-part dosing becomes expensive or tedious to manage. Size it conservatively, use quality media like ARM Coarse, and invest in a decent CO2 regulator with a solenoid. Get your effluent pH stable between 6.5 and 6.8, test alkalinity daily for the first two weeks, then let it run. Once dialed in, it requires less hands-on time than any other supplementation method at the same scale.