How to Mix & Dose BRS Pharma Kalkwasser - Using Calcium Hydroxide In A Reef Tank

Kalkwasser is a hobby term for calcium hydroxide Ca(OH)₂ powder that when mixed with freshwater it creates a solution that will maintain both calcium and alkalinity levels in a reef tank.

When Kalkwasser solution is added to saltwater, the CO₂ in your aquarium water will react with the hydroxide ions in the solution to create bicarbonate and the calcium molecules are allowed to go free. Therefore, you get the benefit of both calcium and alkalinity supplementation with a bonus boost in your pH. The reduction in dissolved CO₂ from this reaction causes pH to rise, and rise very quickly. 

Ca(OH)₂ = Kalkwasser

OH^- + CO₂ = HCO₃ = Bicarbonate

OH^- + HCO₃ = CO₃^-- + H20 = Carbonate + Pure Water

Because you get such an immediate rise in pH, Kalkwasser needs to be dosed very slowly into the aquarium in order to supply and create enough calcium and alkalinity without causing a dangerous spike in pH. Hobbyists have developed a few different ways to administer Kalkwasser effectively over the years including using the ATO system, via a dosing pump, and there are even specific reactors made for administering kalkwasser.

How To Dose Kalkwasser With An ATO

Easiest to Implement

Using your existing ATO is the easiest choice because most of you are already using an ATO so no additional equipment is required. This works great but you are not able to control the dosage directly and you have to ensure your ATO pump avoids sucking up the white precipitate and clogging up the tubes. The idea is to control the concentration of the solution as necessary to keep your tank's alkalinity and calcium levels balanced.  

Add up to 1 1/2 teaspoons per 1 gallon of fresh top-off water in your ATO reservoir. 

1. 1. Start by adjusting your Calcium, Alkalinity, and Magnesium levels to where you want to keep them long term.
   2. With a clean top-off reservoir, fill the container with RO/DI water.
   3. Depending on your demand for calcium & alkalinity, you can add up to 1.5 teaspoons of BRS Pharma Kalkwasser to the reservoir.
      • Low Demand - 1/2 tsp per gallon of fresh water.
      • Medium Demand - 1 tsp per gallon of fresh water.
      • High Demand - 1 1/2 tsp per gallon of fresh water.
   4. Mix the kalkwasser powder into the top-off water with the least amount of stirring possible. Once it is fully mixed, do not mix again, over-mixing will cause CO2 to enter the solution making the kalkwasser less potent.
   5. Let the kalkwasser solution sit for 30 minutes before using.
   6. Activate your ATO and monitor the pH of your aquarium as the kalkwasser solution is being dosed, not exceeding increases of 0.20 pH.

Your ATO should be pumping water into a high flow area of your sump or display tank to help prevent precipitation. Adding kalkwasser too fast may result in precipitation rendering it ineffective. Keep your ATO pump an inch off the bottom of the reservoir to avoid sucking up the precipitation at the bottom.

Monitor your aquarium's calcium and alkalinity levels daily for the next few days, and adjust the strength of the solution as necessary.

Refilling your top-off reservoir will be necessary, and it is as simple as refilling your container with fresh RODI water and adding enough kalkwasser per gallon of water to keep your calcium and alkalinity levels stable. It is recommended to clean the reservoir regularly as precipitation will form on the bottom but isn't necessary after every refill.

How To Dose Kalkwasser Using A Dosing Pump

More Effort, More Control

One of the best approaches 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 we 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.
   • Low Demand - 1/2 tsp per gallon of fresh water.
   • Medium Demand - 1 tsp per gallon of fresh water.
   • High Demand - 1 1/2 tsp per gallon of fresh water.
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, and 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.

Using A Kalkwasser Reactor or Stirrer

Most Consistent Method

There are many different styles of kalkwasser reactors on the market, and most of them function using the same principle. Fresh RO/DI water is fed into an input port on the reactor where the water is either mixed with a pump or a stir bar to create a saturated solution that will be then pushed onward to your aquarium. Most reactors will be able to handle a few cups of kalkwasser, but the capacity will vary between different models.  In any case, you typically will have enough kalkwasser so that it will only require refills every 1-2 months which is why they are so convenient. You also have the benefit of ensuring the kalkwasser is always at saturation for consistent results.    

Learn More: How A Kalkwasser Reactor Works Diagram