kalkwasser reactor examples

Before diving into how to set up a kalkwasser reactor, a quick review of what we discussed our previous article on what kalkwasser is and the many benefits of using it in your reef tank.

As a quick refresher, corals need the right balance of calcium and alkalinity (bicarbonate) to build new skeleton and tissue. If there is an imbalance of either calcium or alkalinity it can inhibit coral growth and make it harder to maintain pH. The goal is to keep calcium at 380-450 ppm and alkalinity at 7-11 dK.  While you can test and add calcium and alkalinity supplements separately, it requires careful dosing and retesting to keep the levels in balance. Many reef aquarists like kalkwasser because it adds both calcium and alkalinity at the same time and in the right ratio. Kalkwasser is German for limewater. Kalkwasser is made by dissolving calcium hydroxide Ca(OH)2 in water. When added to saltwater, calcium hydroxide increases calcium, alkalinity and pH levels.

Dosing Kalkwasser Manually

dosing kalkwasser manually as oppose to using a reactor

It takes some effort to mix up kalkwasser and manually add it to your reef. You’ll have to dissolve the powder in water and allow the undissolved solids to settle. You dose by adding the clear solution to your aquarium. Many aquarists use kalkwasser in ATO systems for topping off the tank. You can’t simply pour it in all at once. Kalkwasser has a pH of about 12 and must be added in small amounts so the pH does not rise too quickly. Kalkwasser requires a closed storage container to prevent it from degrading in strength. The good news is there’s an easier way to dose, with a kalkwasser reactor.

What’s a Kalkwasser Reactor?

A kalkwasser reactor is a sealed chamber that holds the kalkwasser and undissolved calcium hydroxide powder. The reactor has a small circulation pump that periodically stirs the kalkwasser solution. This keeps it fresh, full strength and ready to use. No more mixing and pouring off the clear liquid for top-offs. Here’s the automatic part. The inlet side of the reactor is connected to a freshwater reservoir and top-off pump. The ATO senses when water is needed and sends it through the reactor. Saturated kalkwasser is sent from the reactor into the aquarium or sump through a return line. It couldn’t be easier!

Kalkwasser Reactor Set Up

In addition to the reactor, you’ll need a freshwater reservoir and auto-top-off system. A reservoir can be water jug, a bucket, or a specified ATO reservoir like the ones from Trigger Systems or Innovative Marine. An ATO system is ideal because it senses when top-off water is needed in the tank. It will automatically pump a small amount of kalkwasser into the aquarium or sump. The reactor’s circulation pump blends the freshwater with the calcium hydroxide powder, to make fresh kalkwasser.

diagram of how to set up a kalkwasser reactor
IceCap Kalkwasser Mixing Reactor setup example

Another way to dose kalkwasser is via a dosing pump and reservoir, as seen in the above image of how to set up a kalkwasser reactor. This would be the preferred method since an ATO system can be affected by different factors like temperatures, clogged filters, sensor malfunction, etc. topping off the aquarium reservoir of kalkwasser at different rates. Using a dosing pump will keep the dosing more consistent than using your ATO system in comparison - dosing consistency contributes to environment stability.

Benefits of Using a Kalkwasser Reactor

There are several advantages to running a kalkwasser reactor on your reef aquarium. The reactor automatically keeps the solution saturated with calcium and alkalinity. You won’t have to worry about the kalkwasser losing strength and throwing off the calcium to alkalinity ratio. The reactor periodically stirs up the undissolved powder, keeping the solution saturated with calcium. With an ATO or dosing pump, kalkwasser is automatically dosed in small amounts. This prevents pH swings and keeps the water chemistry stable. You won’t have to use a slow gravity drip or stand at the aquarium carefully pouring kalkwasser into the tank.


Maintaining stable calcium and alkalinity levels is key to good coral growth. Adding kalkwasser is a time-tested way to keep calcium and bicarbonate levels optimized while stabilizing the pH. Using a kalkwasser reactor just makes it easier because it automates the task of daily dosing and maintenance of the kalkwasser solution. We offer several styles of kalkwasser reactors. Be sure to check them out and see which design has the features and size you need for your reef aquarium.

10 Frequently Asked Questions about Kalkwasser