Calcium Reactor (Korallin / GEO)
Korallin / GEO
A chamber of crushed coral skeleton (aragonite) media bathed in CO2-acidified water at about pH 6.5, dissolving the media to release calcium, alkalinity, and trace elements in seawater's own proportions.
Why it mattered
Calcium reactors reached the US in the early-to-mid 1980s through Dupla and Albert Thiel, and Tunze produced the first CO2-controlled unit in 1989. For SPS-dominated tanks burning through alkalinity, the reactor replaced daily dosing with a slow, continuous dissolution of old reef itself.
The chemistry is elegant: the media is coral skeleton, so it releases the same element ratios corals consume, including strontium and magnesium traces that two-part dosing adds separately.
German Korallin units like the 1501 and American GEO reactors became the keeper standards. Run through a quality peristaltic feed pump, a reactor holds alkalinity steady for months at a time, which is why large SPS systems still start with one.
Key specs
- CO2 lowers chamber pH to ~6.5 to dissolve aragonite media
- Releases calcium, alkalinity, and trace elements together
- Tunze built the first CO2-controlled reactor in 1989
- Korallin 1501 and GEO reactors are the classic exemplars
- Best paired with a continuous-duty peristaltic feed pump
Keeper notes
- Keepers warn that media turns to mush if chamber pH falls much below about 6.5 — control the CO2, not just the effluent rate.
- A Masterflex-style peristaltic feed pump is the upgrade owners call transformational for set-and-forget stability.
- The standing tradeoff is display pH: reactor effluent pushes tank pH down, and many keepers pair the reactor with kalkwasser to balance it.