Calcium Carbonate Reactor

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Calcium reactor media and cylinder.jpg
Empty calcium reactor (left), with media (right, calcium carbonate) and carbon dioxide cylinder (center).


Calcium reactors are used to add calcium and carbonate (which adds to the alkalinity) to tank water by means of dissolving calcium carbonate eg coral skeletons. They do this by using carbon dioxide mixed with tank water in a sealed chamber to lower the pH of the water to a level where the skeletons/media dissolves. This water is then dripped back into the tank boosted with calcium and alkalinity.

There are various designs from the simple DIY to fully controlled super expensive.

See also calcium hydroxide reactor, which is another type of calcium reactor.


The calcium reactor has to contain a substrate of calcium carbonate to act as the source of calcium and alkalinity(carbonate). Suitable materials to utilise are:

  • coral sand
  • crushed coral
  • marble
  • commercial calcium reactor substrates (which are simply coral sand). Quality brands currently available in Australia include Aquamedic, CaribSea, Rowa and Tunze.

The thing to be careful with substrates is that they actually contain only calcium carbonate with trace amounts of other things. If it has high levels of phosphate, then that will add considerable amounts of that into the system. For this reason, oyster shells are a bad thing to be used, but most coral sands are OK.





Do I need a calcium reactor?

If you keep clams and stony corals, and have trouble keeping up with the calcium carbonate demand by using kalkwasser then you need a calcium reactor. Alternatively, it is a lazy mans device for adding calcium carbonate, and if you travel a bit it can be very handy to have.

How do I plumb in a calcium reactor?

For the Korralins, setting up properly entails using either a feed from the return or a siphon from the weir to drive water into the unit. Then use the outlet that comes out of the top as the outlet - prevents any bubbles from forming - rather than the aufgang.

How do I tune a calcium reactor?

Tuning is often confusing to those new to calcium reactors. The objective is basically to ensure the effluent has a good amount of calcium carbonate. In order to dissolve the calcium carbonate the pH of the effluent has to be at least 6.9 for reactor media, or 6.7 or lower for crushed marble. The effluent pH is essentially a measure of the chamber pH which needs to be low enough to dissolve the media. This is achieved by a balance of carbon dioxide going in, and the effluent drip rate. A good starting point for many reactors is roughly 1 bubble every ten seconds, and a drop a second of effluent. This can obviously be scaled up to suit needs. A second chamber can be added for greater efficiency and increasing the effluent pH before being added back into the system.

Pros and cons of a calcium reactor