Kalkwasser reactor

Kalkwasser is the hobby term for a Calcium Hydroxide that is intended to be mixed into a solution and slowly added into your aquarium to maintain your calcium and alkalinity levels while also increasing pH. Kalkwasser comes in powder form and is available from a wide variety of different manufacturers.

The powder is mixed with RO/DI water, and after allowing the solution to settle, a clear solution will remain with some white sediment leftover that sinks to the bottom. The clear solution on top is slowly added to your tank and will maintain both calcium and alkalinity. You can administer the solution in a number of ways including a dosing pump, via your ATO, or using a Kalkwasser Reactor.

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 or stirrer which is designed to constantly mix Kalkwasser powder with incoming RO/DI water to maintain the 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.

**Pro-Tip: Kalkwasser in general is best used on tanks of 40 gallons or larger just because of the effects on pH. Smaller tanks just change faster and will feel the effects quicker. There are some DIY methods out there involving the use of vinegar in your solution to help negate the effects on your pH but at that point, using a two-part solution instead would probably be an easier route.

Kalkwasser Reactors & Stirrers

Kalkwasser reactors have been designed in a couple of different ways; there are some which utilize a powerhead to mix the solution, some that utilize a motorized stirring rod known as a "Kalkwasser Stirrer", or ones using a magnetic mixer. In the diagram above, the Ice Cap Magnetic Stirrer is described. 

Each of the reactors is somewhat unique and operates a little differently but most of them will require a timer to dictate the frequency of stirring. The rate of dosing is best controlled independently of the stirring itself with a programmable dosing pump but you could technically use a powerhead on a timer or even your ATO pump to dose through a reactor.  Just reference the particular manufacturer's instructions for the plumbing, necessary equipment, and delivery options. 

How It Works

The reactor is filled with a significant amount of kalkwasser powder which is then periodically stirred with incoming freshwater from your ATO reservoir to ensure every dose is as concentrated as possible. You should use a dedicated dosing pump or small powerhead to operate the Kalkwasser reactor because running the unit in line with your ATO pump does make it difficult to meter the additions.

In concept, you simply add more kalkwasser powder to the container as needed (weekly/monthly) as opposed to manually mixing the solution every few days. In reality, this is true but reactors will also require at least a monthly cleaning and flush to get rid of sediment build-up and prevent any kind of clogging.  

Of course, you will need to monitor your ATO separately as it will run less often when using a dedicated Kalkwasser Reactor as described above.

Manual Mixing & Dosing

You can take a more manual approach to Kalkwasser and just mix some solution manually and dose it into your aquarium without a reactor. The downside here is you just have to mix new solution every few days at the correct concentration but this does work.  It certainly means fewer power cords and wires, you won't have to fuss with cleaning out a reactor, and you're less likely to experience clogging. 

How to mix kalkwasser manually


Kalkwasser solution can be mixed at varying concentrations depending on your particular tank's calcium and alkalinity demand. You can check out the BRS Kalkwasswer Instructions page for more detailed mixing instructions but you essentially start with a lower concentration and increase as necessary to maintain your tank's calcium and alkalinity levels. The maximum concentration of kalkwasser powder is 1.5 - 2 tsp per gallon freshwater.

  • Low-Demand Tank: 1/2 tsp per gallon of freshwater
  • Medium-Demand Tank: 1 tsp per gallon of freshwater 
  • High-Demand Tank: 1.5 tsp per gallon of freshwater

Upon adding the powder to the freshwater, agitate it for 30-60 seconds, let it settle for 30 minutes and a white precipitate will form at the bottom of the container. Then dose only the clear solution that remains on top. 

**Pro-Tip: Kalkwasser solution can be "over-mixed" in that frequent stirring of the solution will diminish the concentration after you have already mixed and let it settle. So the idea is to mix it once, let it settle, without any additional agitation. You always want to store mixed kalkwasser solution in a sealed container too, it's the exposure to CO2 in the air that causes the solution to diminish.  

Dosing Pump

The best approach is to slowly drip the solution into your tank using a high-quality dosing pump. In days past, some aquarists would set up a gravity drip method without a pump but this tends to be a little unreliable and would only recommend this gravity drip approach if you're in a pinch.

The key to kalkwasser delivery is very slow additions, in small amounts, throughout the entire day. Too much Kalkwasswer solution at one time will dangerously spike your pH levels and/or cause precipitation in your tank. If you can achieve a very slow metered drip with your dosing pump, you're going to get the best results.

  1. Test and adjust calcium, alkalinity, and magnesium levels before dosing kalkwasser.
  2. Mix your kalkwasser solution and store it in a sealed container.
  3. Connect and program your dosing pump to deliver the solution into a high-flow area of your tank.
  4. Start with 1 drop every few seconds.
  5. Monitor pH closely over the following few hours/days, never allow for pH changes greater than 0.20 pH.
  6. Test calcium and alkalinity levels daily and adjust the dosage rate and/or concentration of the solution accordingly without spiking pH.

It's also safe to only dose Kalkwasser at night in an attempt to combat the pH drop we all experience when the lights go out and avoid overshooting the pH during the day.

You are dosing freshwater into your aquarium separate from your ATO which means it could potentially affect your salinity levels and how often your ATO refills the tank. Just be mindful of this, watch your salinity levels, and monitor the ATO separately when dialing in a dedicated Kalkwasser drip.

Directly Via An ATO

Another popular route is to mix Kalkwasser powder in the ATO reservoir directly and allow the ATO pump to deliver the solution. This does work but the problem here is you cannot control the rate of dosing and you have to deal with the white precipitate leftover from mixing which can clog your pump and should not be dosed into the aquarium.

This is a simple route to take because most of us have an ATO system running on our tanks so no additional equipment is necessary which is nice. You just have to monitor pH levels closely and adjust the concentration of the solution in your reservoir so as not to spike that pH too much.  The same rules apply, keep the solution stored in a sealed container, do not over-mix or agitate it too much, and always dose into a high flow area to avoid precipitation.

  1. Test and adjust calcium, alkalinity, and magnesium levels before dosing kalkwasser.
  2. Mix and settle the appropriate amount of kalkwasser powder into your ATO reservoir. In this case, it's best to start with the lowest concentration.
  3. Raise the dosing pump at least 2" off the bottom of the ATO reservoir so as to avoid sucking up the leftover white precipitate.
  4. Turn on your ATO and monitor pH very closely over the following 24 hours. Never allow for pH changes greater than 0.20 pH.
  5. Adjust the concentration of the solution necessary to maintain your tank's calcium and alkalinity levels.

Technically, you could alternatively mix a new solution every few days in a separate container. Then attempt to carefully siphon only the clear solution into your ATO reservoir, leaving the white precipitate behind. This does add a considerable amount of extra effort and most hobbyists won't take it this far.

If you really want to automate the process and make it easier, consider a Kalkwasser Reactor or Stirrer instead with a dedicated delivery pump, especially in larger aquariums that require a considerable dose.