The life inside your reef aquarium is constantly using up the calcium and carbonates (dKH) in your tank water. The rate of uptake varies from tank to tank - Calcium and alkalinity consumption largely depends upon the type and quantity of corals you keep in your aquarium.

Reef tanks that are predominantly filled with hard corals—like Acropora, Montipora and Euphyllia, for example — will have a much greater calcium and alkalinity demand than an aquarium filled with soft corals, such as leathers, mushrooms and xenia.

Fortunately, there are a few different ways we can replenish both calcium and alkalinity levels in a reef tank. One the most popular and easiest approaches is by dosing kalkwasser (or simply "kalk" or "limewater").

We receive a lot of questions from hobbyists curious about kalk, so we put together a list of the most frequently asked questions to answer your inquiries and help you decide if using kalkwasser is right for you!

Shop Kalkwasser

1. What is Kalkwasser?

The famed Two Little Fishies company defines Kalkwasser as "the German term for limewater, a saturated solution of calcium hydroxide in water."

Calcium hydroxide—Ca(OH)2—is sold in a powdered form for aquarium use. Most of the top supplement manufacturers produce and package their own versions of Kalk including Bulk Reef Supply! The purpose of dosing kalkwasser is to maintain optimal calcium and alkalinity levels in a reef aquarium.

To dose Kalkwasser, you must first mix the fine calcium hydroxide powder with fresh RO/DI water. This produces a clear liquid solution with some solid white residue leftover that falls to the bottom of the mixing container. You can then dose/drip the clear Kalk solution into your aquarium to help maintain calcium and alkalinity levels.

It should be noted that Kalkwasser solution has a very high pH, it must be added slowly or in small doses to the aquarium in order to avoid dangerous pH spikes.

2. Which should I use - 2-part supplements, a calcium reactor, kalkwasser, or balling method?

There isn't a right or wrong answer to this question because every aquarium and tank owner is different.

Kalkwasser is certainly an economical and easy-to-use solution: you can manually dose it, have it delivered via dosing pump or use a Kalk reactor, each of which has different price points and advantages. You get a nice pH boost and it can handle low to medium demand mixed reef aquariums with a moderate population of stony corals. Some might say Kalkwasser is messy because you have to mix the fine powder and deal with the residual white precipitate. You can usually get away with a once-weekly cleaning and refill of your kalkwasser container. 

Calcium Reactors provide a very consistent level of calcium and alkalinity in your tank and are essentially an automated solution without any regular maintenance required once you have dialed the reactor into your tank's needs. It is also the most expensive and complicated option for supplementation and is great for SPS dominant and/or heavy demand mixed reef aquariums.  

2-Part Solution is the most popular because it is extremely easy to use and affordable for most average reef tank owners. Depending on the exact brand of two-part you choose, there could be a pH boost or not. It really just depends on the particular ingredients said manufacturer is using. There technically is no limit with 2-part dosing but you can experience a drift in ionic balance when dosing a significant amount of two-part solution for heavy demand reef tanks.  Sometimes called a "salinity creep", this happens because of an excess of chloride ions building up in the water. Water changes will combat that imbalance but with heavy dosing, it can become problematic over time. 

The balling method is similar to two-part but takes into account the salinity creep we talked about and requires dosing of 3-4 different solutions. It is the most comprehensive solution for medium to high demand situations that takes into account trace elements and ionic balance. 

3. Can I use kalkwasser and a calcium reactor at the same time?

Without a doubt, yes. If your calcium reactor is having difficulty keeping up with the demands of your tank, dosing Kalk can pick up the slack. Experiment with stronger and weaker Kalk mixes so you can dial in the best dosing regimen for your tank to keep levels stable.

Another great benefit here is that calcium reactors tend to suppress pH levels in the tank while kalkwasser boosts pH. Using a combination of a calcium reactor during the day and kalkwasser dripping at night can result in a very stable pH level throughout the day.

Kalkwasser Reactor

4. How do I dose kalkwasser? Is one approach better than another?

First and foremost, Kalkwasser needs to be dosed slowly. Kalk has a very high pH: if too much is added at once, you may raise the pH of your aquarium water to dangerous levels.

Technically speaking, you can set up a gravity drip, add kalkwasser powder to your top-off water or use a dosing pump to deliver the Kalkwasser solution. In the case of the dosing pump, you can either use a separate kalkwasser stirrer or reactor or simply mix the solution in a bucket and dose it directly using the dosing pump.

Over the years, we have found that a gravity drip approach can quickly clog up, and adding Kalkwasser to your top-off water is hard to dial in given the varied rate of evaporation. That leaves using a dosing pump for the most ideal delivery route.

Kalkwasser reactors replace the need for manually mixing kalkwasser solution in a separate container which is typically done on a weekly basis. The Kalk stirrer or reactor chamber holds Kalkwasser powder that is automatically mixed as freshwater passes through it. The benefit here is you will get maximum concentration out of your solution with every dose and it requires less frequent maintenance compared to mixing a batch of kalkwasser solution manually. You also get the benefit of controlled dosing using a programmable dosing pump.  

The downside of a reactor is it can be messy and depending on the exact design, doesn't really save you much headache. While you do have some options with kalkwasser dosing, most modern hobbyists ultimately find that manually mixing solution on a weekly basis then delivering via a dosing pump daily is the easiest approach.

5. Why should I dose kalkwasser at night?

In a reef aquarium, you often see a swing in pH levels throughout the day. pH levels are highest in the middle of the day and lowest at night which happens because of CO2 levels in the water.  When corals photosynthesize during he day, they take up dissolved CO2 from the water which increases the pH. At night, the photosynthesize stops, therefore more dissolved CO2 is in the water which suppresses the pH.

Dosing Kalkwasser at night can combat that pH drop and reduce the dissolved CO2 levels. 

Does this mean it is bad to dose during the day? Not necessarily, especially if your tank needs that much calcium and alkalinity. You can dose kalk-solution during the daytime but you will want to make sure the amount being dosed doesn’t cause the pH to rise too high. A very slow and regular drip is the best approach, for example, you might add 1 mL of solution every few minutes.  This spread-out addition minimizes dramatic spikes in pH but gives you an overall boost. 

6. How long after I mix up my kalk-solution I use it in my tank?

This will vary depending on how much you mix, but you will see a clear liquid within an hour that is ready to be used in most cases. Once mixed, you just have to wait for that sediment to separate and settle on the bottom before you dose it. Do not agitate the solution thereafter either because that kicks up the white sediment and can also reduce the potency of the solution.  You want to minimize gas exchange with kalk-solution so keep it in a sealed, undisturbed container. 

7. Can I reuse leftover undissolved Kalk powder or do I need to mix a new batch every time?

It is best to just mix a new batch once a week unless you're using a kalkwasser stirrer. Start with 1/2 - 1tsp of Kalkwasser powder per gallon of fresh water in terms of concentration and adjust to a maximum of 2 tsp per gallon of freshwater if your tanks demand it. The solution's maximum concentration is right about 2 tsp of kalkwasser powder per gallon. Anything more and the leftover calcium hydroxide will settle down alongside that white sentiment. Technically speaking you could reuse whatever leftover calcium hydroxide remains in that sediment but if you're manually mixing the solution without a reactor, it really has no benefit and is hard to gauge the concentration. It just becomes messy and clogs up tubing/fittings as well.  

A kalkwasser reactor works differently and is designed to work with a supersaturated solution which reduces the need for regular additions of powder. You add far more than the maximum concentration of kalk-powder to the reactor. The reactor does the work, keeping that solution at maximum concentration while the pump is always feeding freshwater to replace what you dose. In this case, you should only have to clean the reactor once a month or so to dump out the build-up of sediment that ultimately occurs. 


Mixing Kalkwasser

8. How do I know how much Kalk powder to mix with fresh water for my tank? How do I know if I am using enough/too much kalk?

The simple answer is to test your water parameters. For mixing, start with a lower concentration (1/2 to 1 teaspoon per gallon of fresh RO/DI water) and monitor your tank's parameters over the next couple of days. Adjust the dosage into your tank to maintain stable levels over the following few days. Be mindful of that pH level and if you see pH rise above 8.4 or more, you are dosing too fast.

If you cannot maintain levels, you can increase the concentration to a maximum of 2 tsp per gallon of fresh water. That is the maximum amount of calcium hydroxide you can mix into a solution.

From that point, you just have to manipulate how fast the solution is dosing into your tank in order to maintain your levels without spiking pH. A constant drip is the best approach to avoid those spikes. For example, trying to dose 20mL of kalkwasser solution all at one time on the hour, every hour, will often cause a dangerous pH spike. Slow drip is the key.

9. Are there any downsides or possible negative effects from dosing Kalk?

Due to the caustic nature of kalkwasser, it can cause major issues in your aquarium if overdosed. Specifically, it can raise your pH to dangerous levels or cause a precipitation event if too much calcium or carbonate is added to the tank.

If you are dosing with a dosing pump or other automated system, you may want to use an aquarium or pH controller to stop dosing in the event the pH level rises too high.

10. I have a nano reef tank, can I dose Kalk?

We generally do not recommend dosing Kalk in nano aquariums. Changes in water chemistry occur more quickly in nano aquariums due to their small size and using Kalk could lead to a dangerous pH spike.

That said, if you're an experienced reef tank owner and feel that dosing Kalk is the best way to maintain your calcium and alkalinity, by all means, proceed. Mix a very mild solution using 1/4 - 1/2 tsp of Kalk powder per gallon of fresh water and monitor pH very closely to mitigate the risks.