Calcium Hydroxide Reactor

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A Calcium Hydroxide Reactor is also commonly called a Nilsen Reactor or Kalkwasser Reactor.

A calcium hydroxide reactor is a very simple piece of equipment that dissolves calcium hydroxide into pure water. The solution has a high pH, around 11-12, and at this level has a significantly high solubility of calcium. The saturated calcium hydroxide solution is is then added into the reef aquarium in a slow manner, typically as the evaporation top up. It adds both calcium, Ca2+, and carbonate, CO32-, (indirectly via the hydroxide, OH-) to the system, increasing both calcium and alkalinity levels.

The calcium hydroxide reactor adds 1 meq/l alkalinity for 20ppm calcium, which is the same ratio that is utilised by calcifying organisms. This is the reason why it is such a good technique for maintaining the calcium and alkalinity levels, it adds them in the same ratio in which they are used up. This is also why it cannot be using for fixing imbalances of calcium/alkalinity, a separate additive (calcium chloride or sodium bicarbonate) has to be used for that or significant water changes.

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





Pros and cons of a Calcium Hydroxide Reactor



  • since it produces kalkwasser to replace evaporation, depending on the rate of evaporation in your tank, kalkwasser may not provide enough calcium and alkalinity to maintain the systems demands.