ReefDeck
Open the free logbook
Calcium Reactor vs Dosing: Which Is Right for You?
A calcium reactor dissolves calcium carbonate media with CO₂-enriched water, releasing alkalinity, calcium and magnesium together in roughly natural seawater ratios — it suits larger, heavily stocked SPS tanks where consumption is high and constant. Two-part dosing uses separate liquid additives, is cheaper to start, simpler to run, and better suited to smaller or lightly stocked tanks. Both need the same thing to work well: regular testing, because neither method is truly "set and forget."
Once a reef tank moves beyond a handful of soft corals, alkalinity, calcium and magnesium start disappearing faster than water changes alone can replace them. At that point most reef keepers choose between two supplementation methods: a calcium reactor or two-part liquid dosing. Both exist to solve the same problem — replacing what corals consume — but they do it in very different ways, and the right choice depends more on your tank's demand than on which method is "better."
How a calcium reactor works
A calcium reactor is a sealed chamber filled with calcium carbonate media — usually crushed aragonite or a similar reef-safe rock. A small stream of CO₂ gas is injected into the chamber, which lowers the pH of the water inside and dissolves the media. That CO₂-enriched, mineral-rich water then drips slowly back into the tank as effluent, adding alkalinity, calcium and, usefully, magnesium and trace elements in something close to their natural seawater ratio, since they were locked in the same rock to begin with.
Because the output ratio is fixed by the media rather than by the reef keeper, a well-tuned reactor rarely causes the kind of imbalance that can happen when dosing additives separately. The trade-off is that tuning it takes patience: the CO₂ needle valve, effluent drip rate and reactor pH all need adjusting together, and each change takes a day or two to show up in your test results.
How two-part dosing works
Two-part dosing uses two separate liquid solutions — one alkalinity-based, one calcium-based — added directly to the tank, usually with a dosing pump running several small doses a day rather than one large one. Some reef keepers add a third solution for magnesium when needed. Because the two solutions are dosed independently, the reef keeper controls the ratio directly, which makes dosing easy to start and easy to adjust on short notice.
The main limitation is scale. Two-part solutions are concentrated, but a heavily stocked SPS tank consuming a lot of alkalinity daily can need a genuinely large volume of solution to keep up, which gets expensive and means refilling dosing containers often. Magnesium and trace elements are not automatically included and need separate attention.
Which suits your tank
There is no single correct answer here, and plenty of successful tanks run either method, or even both at once — a reactor for baseline alkalinity and calcium, topped up with occasional dosing when a batch of new frags briefly pushes consumption higher. The table below is a starting point based on tank size, coral load and how much hands-on upkeep you want, not a hard rule.
| Situation | Better fit | Why |
|---|---|---|
| Small to medium tank, moderate coral load | Two-part dosing | Lower upfront cost, simple to set up, easy to adjust as the tank changes |
| Large, heavily stocked SPS tank | Calcium reactor | Cheaper per unit of alkalinity replaced at high demand; media lasts far longer than liquid dosing volumes |
| Reef keeper who travels or wants low weekly upkeep | Calcium reactor | Once tuned, a reactor runs for weeks between media top-ups; dosing containers need more frequent refilling |
| New or growing tank, demand still changing | Two-part dosing | Doses adjust instantly; a reactor's effluent rate takes longer to retune safely |
| Tight budget or limited plumbing space | Two-part dosing | No CO2 cylinder, regulator, reactor vessel or extra plumbing to install |
Cost and maintenance in practice
A reactor has a higher upfront cost: the vessel itself, a CO₂ cylinder and regulator, media, and usually a dosing or return pump to drive water through it. Running costs are low once tuned, since media and CO₂ refills are infrequent relative to the volume of alkalinity produced. Two-part dosing flips that pattern — a dosing pump and starter bottles are inexpensive, but ongoing solution costs scale directly with tank consumption, so a demanding SPS tank on dosing alone can end up costing more per month than a reactor would.
Monitoring is identical either way
Whichever method you choose, the testing routine does not change. Alkalinity should still be tested two to three times a week, with calcium and magnesium checked weekly, regardless of whether the numbers come from a reactor's effluent or a dosing pump's schedule. A reactor that is producing slightly too little alkalinity looks exactly like an under-dosed tank on a test kit — the only way to tell is to log the result and watch the trend.
This is where a lot of reef keepers get caught out: they assume that installing a reactor removes the need to test, when in fact it just changes what you are checking for. A reactor needs its output verified after every effluent or CO₂ adjustment; dosing needs its volume re-checked as biomass changes. Logging each test with a date, whichever method you run, is what turns a one-off reading into a picture of whether your supplementation is actually keeping pace with your corals.
Track either method against your own results
ReefDeck logs alkalinity, calcium and magnesium from every test so you can see whether your reactor or dosing regime is actually holding steady — free, offline, no account required.
Open ReefDeck — it's free → Works on phone and desktop · installs as an app · exports to CSV anytimeFrequently asked questions
Is a calcium reactor better than dosing?
Neither is universally better — a calcium reactor tends to suit larger, heavily stocked SPS tanks with high and constant alkalinity demand, while two-part dosing suits smaller or lightly stocked tanks and is cheaper and simpler to start. The right choice depends on your tank's consumption, budget and how much weekly upkeep you want.
Does a calcium reactor add magnesium too?
Most calcium reactor media contains some magnesium and trace elements alongside calcium and alkalinity, released in roughly natural seawater ratios as the media dissolves. Two-part dosing usually covers only alkalinity and calcium, so magnesium often needs a separate additive.
Do I still need to test if I have a calcium reactor?
Yes. A reactor's effluent rate is a starting point, not a fixed setting, and consumption changes as coral biomass grows. Testing alkalinity two to three times a week and calcium and magnesium weekly is the only way to confirm the reactor's current output is actually keeping pace with the tank.
Is two-part dosing cheaper than a calcium reactor?
It is cheaper to start, since a dosing pump and starter bottles cost far less than a reactor, CO2 cylinder and regulator. But ongoing solution costs scale directly with consumption, so on a heavily stocked SPS tank, dosing can end up costing more per month than a reactor once it is tuned.