Powder kalkwasser examples for mixing with RO/DI to make solution

Corals and other reef-building organisms require calcium and alkalinity to grow. Reef-building corals extract calcium and alkalinity (bicarbonate) from the water to form a calcium carbonate skeleton. Coralline algae also use calcium and bicarbonate to lay down layer upon layer of encrusting algae. Alkalinity also plays a role in keeping the pH stable. So, before getting into kalkwasser and how to mix your own solution, let's take a look at the relationship between the two.

A Balance of Calcium and Alkalinity

Maintaining calcium and alkalinity levels is important in reef aquariums because an imbalance in either one can limit the growth of your corals and make it harder to keep your water chemistry stable. Expert aquarists have found that keeping calcium at 380-450 ppm and alkalinity at 7-11 dKH is ideal for reef keeping.  There are many ways to keep calcium and alkalinity levels within this range, with the most popular method being a 2-part dosing system using individual calcium and alkalinity booster products. With 2-part you must maintain the proper ratio of calcium to alkalinity, which will also require careful testing, dosing and re-testing to make sure both parameters are within range. But there’s another way to add calcium and alkalinity with one product: Kalkwasser!

So, what is Kalkwasser?

If you’re new to reef-keeping you may have never heard of kalkwasser? Kalkwasser is German for limewater. Limewater, AKA kalk, is a highly concentrated solution of calcium hydroxide Ca(OH)2 . It contains about 800 ppm calcium and 100 dKH alkalinity. Adding it to your aquarium will increase both calcium and alkalinity in the right ratio.

How to Mix Your Own Kalkwasser

It’s easy to mix kalk solution. All you need is an aquarium-safe calcium hydroxide powder and fresh RO/DI water. If you’re researching DIY kalkwasser, you may read about buying cheap lime powder at home centers. This isn’t recommended because there is no guarantee as to the purity of the product since it is used for construction purposes. In fact, many of these lime powders won’t work at all, because they are powdered calcium carbonate (limestone). Limestone is insoluble in water. Products like E.S.V. Calcium Hydroxide and Seachem Reef Kalkwasser use high-purity calcium hydroxide powder, ensuring you won’t add anything harmful to your tank. Here are the basics of mixing up kalkwasser:

Mixing kalkwasser solution diagram
Steps in Mixing your own Kalkwasser Solution

  1. Add 1-2 teaspoons of powder to a gallon of RO/DI water (per manufacturer recommendations). RO/DI water works best and allows you to dissolve more powder in the gallon of water.
  2. Stir to dissolve the powder. Do not use an air stone! Undissolved material will settle at the bottom.
  3. Keep the container closed to prevent carbon dioxide from entering the solution. Carbon dioxide will slowly degrade the kalkwasser by forming insoluble calcium carbonate on the bottom of the container.

How to Dose Kalkwasser

Use ONLY the clear supernatant (top solution) and do not allow undissolved material to enter the aquarium. After mixing, the usable clear kalkwasser solution contains calcium and hydroxide, with a pH of about 12. As kalkwasser is slowly added to your tank’s water the hydroxide is converted to bicarbonate (alkalinity). Slow hand pouring, a gravity-drip setup or an automated continual drip-dosing system (which we will discuss in a future article) are all safe ways to increase calcium and alkalinity, and it also stabilizes the pH. Just be aware, if you add too much kalkwasser at one time, the pH will shoot too high then drop down as the pH stabilizes.

Demonstration of using the solution in the auto top off reservoir
ATO Dosing: Carefully add the clear portion of kalkwasser solution to your top-off reservoir. DO NOT allow undissolved material to enter the aquarium

Some aquarists simply top off the tank with kalkwasser every day, which works well with nano tanks that have minimal evaporation. Another option is to dose kalkwasser with an automatic top off (ATO) system. Instead of pouring in kalkwasser at the end of the day, an ATO automatically adds water throughout the day and night. This keeps the aquarium topped off and it acts as a slow dose of calcium and alkalinity. ATOs are especially helpful on medium to large aquariums or tanks that lose a lot of water through evaporation. You can pair your ATO with a reservoir, making it easy to keep an eye of the kalkwasser and mix up new batches when needed. Be sure to test tank parameters to dial in your solution saturation.


One of the most important ways to maximize coral growth is maintaining calcium and alkalinity levels. Using kalkwasser has proven to be an easy and reliable method to increase calcium and bicarbonate levels in a balanced ratio along with the benefit of stabilizing pH. If you’re having difficulty increasing these parameters or just want to simplify your supplement dosing, try kalkwasser!