Reef Octopus Kalkwasser Reactor 101

O ver the last year or so, there has been a revival of Kalkwasser use amongst reef aquarists. 

What was once a forgotten method; reefers are again using Kalkwasser as a sole source of calcium and alkalinity while others are using it as a supplement to a calcium reactor or 2-part dosing regimen.   

In this video we will cover the key benefits of Kalkwasser, touch on some of the dangers associated with dosing it and how to dose it safely and effectively using the Reef Octopus Kalk Reactor.   

Calcium Hydroxide, limewater, or more commonly known in our hobby as Kalkwasser, supplements equal amounts of both Calcium and Alkalinity in one simple solution.

When Kalkwasser is mixed with water it forms a balanced solution of Calcium and Hydroxide ions. The addition of Calcium ions will supplement the calcium level in the tank while the Hydroxide ions react with the dissolved C02 to form Bicarbonate and therefore supplement alkalinity.

A key benefit of Kalkwasser is that when properly added will help maintain elevated pH levels and encourage optimal coral growth.  

Low pH is a common problem for most home aquariums due to high-efficiency doors and windows.  Kalkwasser solution has a natural pH of 12.4 which helps bring your overall aquarium pH up. 

A side perk to Kalkwasser use is its ability to help maintain phosphate levels low by precipitating it out of solution and making it unavailable for algae, cyano and unsightly dinoflagellates.

When it comes to supplementing Calcium and Alkalinty, it does not get any cheaper or cleaner than Kalkwasser. Kalkwasser is very cheap and most Aquarium Sourced Kalkwasser is lab grade with almost no impurities.

Over the years, Kalkwasser was commonly mixed into a fresh water reservoir managed by an Auto Top Off System to slowly supplement Kalk solution throughout the day to replenish evaporated water.

Although this approach is simple, inexpensive and does work, it also has also led to many catastrophic tank crashes with total loss of corals.

If Kalk is overdosed:

  • The aquarium pH can instantly spike to unsafe levels, precipitate out alkalinity and deplete CO2 levels within the water resulting in coral death.
  • Another shortcoming of the ATO method is the dosing fluctuations due to changes in the rate of evaporation of your tanks. Our aquarium’s water evaporates differently throughout the day and more so throughout the seasons. This tends to make it difficult to supplement equal amounts 24/7.
  • Another drawback of the ATO method is most water top off containers are not airtight.  Kalkwasser degrades and looses its efficacy when exposed to air. 
  • Lets not forget that Kalkwasser is very hard on pumps.  Even when the pumps are properly maintained, Kalkwasser will shorten their lifespan drastically.

A more efficient and safer method of adding Kalkwasser to your system is by using the Reef Octopus Kalkwasser reactor.  This reactor is a sealed container that automatically houses your Kalkwasser solution in an airtight environment preserving its freshness and potency. 

RODI water is injected into the bottom of the Reef Octopus Kalk reactor forcing out a highly saturated solution from the output located at the top of the reactor. A slow RPM motor with an attached stirring rod gently sifts the Kalkwasser into solution a few times a day to maintain a continuous source of super saturated Kalkwasser. 

The use of a programmable dosing pump like the Kamoer X-4 or the new X1 pump can provide a highly effective and safe way to feed Kalkwasser into a reef system.

The advantage of using the Reef Octopus Kalkwasser reactor paired with a reliable liquid doser is that it allows for the utmost control to ensure that only a small, predictable amount of Kalkwasser is added to your system at any given time. This aids in preventing a Kalk overdose.

The Reef Octopus Kalk Reactor is available in three different sizes:

  • The smallest model, KS-100 it a 4in by 27.5in tall Reactor for aquariums up to 130 gallons.
  • The KS-150 is a 6in body by 26in tall reactor for aquariums ranging from 100 – 200 gallons.
  • And the KS250, the largest reactor of the group, is a 10in body by 26in tall reactor capable of handling aquariums between 200 and 300 gallons.

For the installation of our Reef Octopus Kalkwasser reactor we will need the following items:

  • Kalkwasser
  • RODI water and reservoir
  • a Reliable Dosing Pump to feed the reactor, and
  • An Appliance Timer or controllable outlet to periodically operate the motor

Place the Reef Octopus Kalk Reactor upright on a flat and stable surface away from high traffic areas and out of reach of children.

Using a barbed adapter that can be found at your local hardware store, connect the flexible tubing included with your Kamoer pump to the reactor’s input located at the bottom of the reactor.  

Connect the other end of the line to your Kamoer’s output.

Now attach a length of tubing to the dosing pump’s input and secure the other end of the line in the fresh water reservoir. If you have one for your systems ATO we suggest sharing a single vessel.

Now take another length of tubing and connect it to the output located at the top of the reactor.  Secure the line to your sump in a high flow area away from pumps and heaters.

Loosen the screws on the lid enough that the top will rotate slightly counter clockwise and lift off.   If needed the screw under the lid can be unscrewed to detach the motor with lid from the Mixing rod for easy filling and maintenance.

Fill the reactor ¾ of the way with RODI water. 

Next add Kalkwasser powder.

  • If you have a KS-100, add 90-120g of Kalkwasser
  • For the KS-150, add 120-150g of Kalkwasser
  • The KS-250 will require 150-180g of Kalkwasser.

Avoid adding excessive amounts of Kalk to the reactor, as it will cause the Kalkwasser to cake up at the bottom and burn out the motor.   



Please make note of your Kalkwasser warning label. Kalkwasser is highly caustic and extreme care should be used when handling it.

Using a spoon, manually premix the water and Kalkwasser into a “Kalk slurry” and top off the reactor with RODI water till it reaches the upward elbow located on the output inside the reactor. Wait about an hour for the Kalkwasser to settle before dosing.

Fully saturated kalk solution will be clear.  



Milky, undissolved Kalkwasser should never be added to the tank as it can quickly spike pH and alkalinity to fatal levels.

Replace the reactor lid with attached stirrer rod into the designated spots with the screws and rotate the lid slightly clockwise. Tighten each screw to lock and seal the lid to the body.

Plug the power adapter into a timer or controllable outlet and set it to mix for 15min every 2 hours.


The stirrer should never be left on or used 24/7.

Before we begin dosing, we must test our Calcium and Alkalinity of our aquarium water and if needed supplement to our desired levels prior to the using the Kalk Reactor.


DO NOT use Kalkwasser for this purpose, as it will significantly impact your aquariums pH and Co2 Levels.  

We suggest using separate solutions of calcium and alkalinity.

After the slurry has settled we can begin dosing our Kalk.  It is much safer to dose less and test than to overdose.  Lets start by priming the reactor and lines by continuously running the dosing pump till liquid exits the output line. Then set the doser to disperse half the volume required to replenish your systems evaporated water on a daily basis. So if my 220gal system evaporates 5 gallons of water a day I will set my doser to disperse 2.5gallons in small amounts throughout the day.

We highly recommend you carefully monitor your pH when adding Kalkwasser. If your pH increases more than 0.2, decrease the amount of Kalk added.

In addition, Testing of Calcium and Alkalinity will help you dial in your dosage to the exact amount needed to sustain the demands of your system.

FAQ File

Here are answers to some commonly asked questions:

How often do you have to refill and clean the reactor?

It really depends on the systems demand. For most reefers the Kalkwasser should be topped off every couple weeks or as needed. It would be a good idea to clean the reactor on a monthly basis.

What happens if Kalkwasser is overdosed or dosed too quickly?

Your pH will spike and will likely result in coral death.

What if my Kalk dosing is not keeping up with my aquariums demand for Calcium and or Alkalinity?

On higher demand systems, you may be required to use 2-part or Calcium reactor to supplement the bulk of your Alkalinity and Calcium needs.

Well, that is all for this video. If you have any questions or issues with the product, don’t hesitate to visit our support portal at  Our friendly support reps are eager to help you with any questions or issues you may have. 

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Author: Carlos

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