Methods

Calcium reactors for reef tanks: how they work

Carbon dioxide lowers the pH in a sealed chamber of aragonite media, which dissolves and releases calcium, alkalinity and trace elements in roughly the proportions corals use. Expensive to buy, cheap to run, fiddly to tune.

Last checked 21 October 2026. How we check

Cost

High up front, low running

Effort

Medium to tune, then low

Suits

Heavily stocked stony coral tanks

Needs

CO2 cylinder, regulator, sump space

How does it work?

Tank water passes slowly through the media chamber, where dissolved CO2 keeps the pH low. The effluent leaving the reactor is rich in calcium and alkalinity.

What pH and effluent should a calcium reactor run at?

Reef Builders (2014) gives a reactor pH of about 6.8 to 7.2. Manufacturer and retailer guides more often say 6.5, within 6.3 to 6.8 (typical, check). CO2 at about one bubble a second is a common start; effluent alkalinity of about 25 to 40 dKH is typical, and you dilute it two or three to one before testing so your kit reads it.

The extra CO2 can lower tank pH. Keepers often feed the effluent through a refugium or algae reactor to use some of it up.

How do I get started?

1. Measure daily alkalinity use first.
2. Fit the reactor in the sump with a CO2 regulator and solenoid.
3. Set reactor pH with a probe, then tune effluent drip rate.
4. Change one thing at a time and wait a few days.
5. Test alkalinity daily while tuning.

What are the common mistakes?

• Changing CO2 and drip rate at the same time.
• No solenoid on the CO2, so a power cut keeps gas flowing.
• Ignoring a falling tank pH.

Where do sources disagree?

Figures from sources read as a search summary only are shown as typical figures: check them before you rely on them. Found a mistake? Tell us how to fix it.