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    Kalkwasser dosing for reef tanks: how and how much

    Calcium hydroxide dissolved in the top-off water adds calcium and alkalinity in balance, raises pH and costs almost nothing. The catch is that evaporation limits how much it can add, and a stuck pump can spike pH.

    Last checked . How we check

    Cost
    Very low
    Effort
    Low once running
    Suits
    Low to moderate demand; tanks with low pH
    Limit
    Only as much as evaporates

    How does it work?

    Kalkwasser (limewater) is calcium hydroxide in RO/DI. Saturated at 25 C it holds roughly 800 to 816 ppm calcium and 40 to 41 meq/L of alkalinity (typical, check). It goes in with the top-off, so the dose is tied to evaporation.

    How strong should kalkwasser be?

    BRS gives three strengths: half, one or one and a half teaspoons per US gallon (3.8 litres), with one and a half as the limit of saturation. Let it settle for 30 minutes and draw from about 1 to 2.5 cm above the sediment. Never let it lift pH by more than 0.2; dose into strong flow, ideally at night; test calcium and alkalinity daily at first.

    How much alkalinity can kalkwasser supply?

    About evaporation times 41 meq/L, divided by system volume (our arithmetic). A 300 litre system losing 3 litres a day can add about 0.41 meq/L, or 1.15 dKH, a day. If your tank uses more, add another method alongside. Measure use first with the daily consumption calculator.

    How do I get started?

    1. Measure daily evaporation and alkalinity use.
    2. Start at half strength in the top-off.
    3. Test pH, calcium and alkalinity daily for the first week.
    4. Raise strength slowly while pH stays inside a 0.2 rise.

    What are the common mistakes?

    • Topping off with kalkwasser from a large reservoir that could dump at once.
    • Stirring up the sediment into the tank.
    • Mixing it with Balling Light.
    • Relying on it for a tank that uses more than evaporation can carry.

    Where do sources disagree?

    Where sources disagree
    TopicWhat sources sayOur reading
    Strength
    • BRS: up to one and a half teaspoons per US gallon
    • Other keepers: always saturated, let the excess settle
    		Start weak; strength matters less than steady delivery.

    Sources

    1. BRS, using kalkwasserPublished 31 August 2022. Read 21 October 2026, read in full.
    2. BRS, auto top-off and evaporation FAQPublished 6 February 2024. Read 21 October 2026, read in full.
    3. Reef Builders, Fauna Marin Balling Light instructionsPublished 11 September 2009. Read 21 October 2026, read in full.
    4. Reef Builders, how and why to dose your reef tankPublished 19 September 2023. Read 21 October 2026, read in full.

    Figures from sources read as a search summary only are shown as typical figures: check them before you rely on them. Found a mistake? Tell us how to fix it.