Kalkwasser is the hobby term for a Calcium Hydroxide solution that is intended to be slowly added into your aquarium to help directly maintain your calcium levels. Kalkwasser comes in powder form and is available from a wide variety of different manufacturers. The powder is mixed with RODI water, and after allowing the solution to settle, a clear solution will remain with some white sediment left over that sinks to the bottom. The clear solution on top is then slowly added into your tank. You can administer the solution a number of ways including manual dosing, gravity drip, via your ATO, dosing pump or even using a plain old timer and powerhead.

In order to get the most out of your Kalkwasser solution, you only want to dose the highly-saturated clear solution into your aquarium. Enter a Kalkwasser reactor which is designed to constantly mix Kalkwasser powder with incoming RODI water to maintain maximum concentration of Calcium Hydroxide at all times. A reactor will also ensure that only the clear solution is dosed into your aquarium and helps to eliminate any risk of accidentally adding the white residue that settles on the bottom.

Kalkwasser Reactors

Kalkwasser Reactor

Kalkwasser reactors have been designed a couple of different ways; there are some which utilize a powerhead to mix the solution, ones that utilize a motorized stirring rod known as a Kalkwasser Stirrer, or ones using a magnetic mixer.

The way a Kalkwasser Reactor works is quite simple as they essentially are just a mixing chamber for RODI water and Calcium Hydroxide. Freshwater is fed into the reactor via your ATO system, dosing pump or gravity feed. Kalkwasser powder is added inside the reactor and is then mixed together with fresh RODI water via the powerhead. The powerhead is attached to a timer to periodically mix the powder and ensure high saturation.

Usually the mixing is set to occur 3-5 times per day for about 10 minutes each time. The clear solution that resides on top is then dripped into your aquarium based on your tanks demand for calcium and alkalinity. If you need more calcium, increase the drip rate! This is the beauty of using a reactor, the solution going into your tank will always be highly saturated which means you will not need to worry about precipitation and constantly adjusting the amount of kalkwasser to mix into solution.

It is recommended that you clean out the residue from the bottom of the reactor on a weekly basis. Simply attach some tubing to the cleaning port, open the valve and turn on the powerhead. Once the reactor is clean, you can add new Kalkwasser and continue normal operation. It is also important to rinse any circulating pumps by removing it from the reactor and removing calcium build up on a monthly basis.

Kalkwasser Stirrer

A Kalkwasser stirrer works pretty much the same exact way as the powerhead driven Kalkwasser Reactors, but utilizes a small motor and stirring rod to mix the solution instead of a powerhead. The benefit of using a Kalkwasser Stirrer is they are a bit easier to clean compared to a powerhead driven reactor. Simply remove the lid and flush the reactor, no powerheads or valves to fuss with.  

Some reactors are equipped with a magnetic mixer powered by a motor attached to a timer which periodically mixes the kalkwasser solution throughout the day. Since the mixing is less aggressive you will probably want to mix the solution a more frequently when compared to a powerhead driven reactor.  Some Kalkwasser Stirrers can be set to run 24/7 but be sure to read through the manufacturer instructions carefully before operation.    

Kalkwasser Diagram

Kalkwasser has long been used by hobbyists as a very economical way to maintain calcium in a reef tank.  Although very effective, it is important to dose correctly and have a thorough understanding of how it should be added to your aquarium. Kalkwasser solution has a very high pH level around 11 and could be very dangerous to your reef tank if dosed too quickly or incorrectly.

Check out our supplemental article Kalkwasser: 10 Frequently Asked Questions and watch our video How to Install and Use a Kalkwasser Reactor to Maintain Calcium & Alkalinity in a Reef Tank