Why not just dose two-part instead?

You can. Two-part is more precise and lets you tune Ca and alk independently. Kalk wins on simplicity (one reservoir + one pump), pH stabilisation (raises pH where two-part drops it), and cost (calcium hydroxide is cheap by the pound). Most reefers run kalk as the baseline and bump with two-part for the demand it can't cover.

Will kalk crash my pH if the dosing pump fails on?

Yes — that's the failure mode that scares people off kalk. Limit your reservoir to one day's evaporation (a 1-gal jug for most tanks), and run the dose pump on a controller-timed schedule with hard mL/min caps. A whole reservoir dumped is recoverable; a whole 5-gallon bucket dumped usually isn't.

Can I use kalk in a tank with low evaporation?

Less effectively. Kalk's Ca/alk ratio is fixed by stoichiometry — you can't add more without overdosing one or the other. Low-evap tanks (<0.5 % of volume/day) often run two-part with kalk as an occasional supplement.

Hot or cold mixing water?

Cold dissolves slightly more kalk than warm, but the difference is small. Use RO/DI at room temp, mix to saturation (you'll see undissolved powder), let it settle 12 h, and draw from the top — never the sediment.

Calculator

Kalkwasser Drip Rate Calculator

Name: Kalkwasser Drip Rate Calculator
Author: reefcalcs

Kalkwasser (saturated calcium hydroxide solution) is the lazy-reefer's perfect dose: every mL replaces evaporation AND adds Ca and alk at exactly the ratio coral takes them up. The trick is matching drip rate to evaporation so you don't dilute salinity. This calculator solves both at once — mL/min for your schedule, and the actual ppm Ca and dKH you're adding so you know whether to also dose two-part.

JavaScript is disabled in your browser, so the interactive calculator below won't run. The math, formula, and worked example are all written out further down this page under “How this is calculated.” Enable JS (or open this page in another browser) to use the live calculator.

Whether you compute by hand or in the calculator, the same caveat applies: verify against your test kit and the linked primary source before dosing. reefcalcs is reference information, not veterinary or engineering advice.

Kalkwasser drip rate

Tank volume (US gal)

Daily evaporation (US gal/day)

Equal to your ATO refill volume.

Drip window (hours/day)

Schedule

Night dosing avoids midday pH spikes.

Drip rate

5.26mL/min

Per hour

315.5mL/hr

Per day total

3,785mL

Matches your evaporation in mL.

Ca added per day

10.4ppm

Alk added per day

0.53dKH

From saturated kalk at 25 °C (~40 dKH per L).

Saturated kalk: ~800 ppm Ca and ~40 dKH per litre at 25 °C (Holmes-Farley, Advanced Aquarist). Drip via a peristaltic or aquadoser into a high-flow area, never directly onto coral.

How this is calculated

Saturated kalkwasser (25 °C, well-stirred):
  Ca  ≈  800 ppm per L of solution
  Alk ≈  40  dKH per L of solution

Drip rate:
  mL/day = evaporation_gal × 3785.41 mL/gal
  mL/min = mL/day ÷ (active_hours × 60)

Daily addition into tank:
  Ca_ppm   = (mL_per_day × 0.8 mg/mL) ÷ (tank_L × 1.025)
  Alk_dKH  = (mL_per_day ÷ 1000 × 40 dKH/L) ÷ tank_L

Safety cap: drip ≤ 1 mL/gal/min during any minute.

The 800 ppm Ca / 40 dKH figures are from Randy Holmes-Farley's calcium-hydroxide solubility data. Real kalkwasser comes out slightly weaker because (a) atmospheric CO₂ converts hydroxide to carbonate over hours and (b) most reefers don't fully stir to saturation. Plan on ~80 % of the nominal numbers in practice.

Drip on lights-off if you have any pH instability. Daytime photosynthesis already pushes pH up; adding kalk on top can push past 8.5 and precipitate. Lights-off drip works as a built-in pH stabiliser: pH drops at night as CO₂ accumulates, and the kalk is right there to nudge it back up.

FAQ

Why not just dose two-part instead?: You can. Two-part is more precise and lets you tune Ca and alk independently. Kalk wins on simplicity (one reservoir + one pump), pH stabilisation (raises pH where two-part drops it), and cost (calcium hydroxide is cheap by the pound). Most reefers run kalk as the baseline and bump with two-part for the demand it can't cover.
Will kalk crash my pH if the dosing pump fails on?: Yes — that's the failure mode that scares people off kalk. Limit your reservoir to one day's evaporation (a 1-gal jug for most tanks), and run the dose pump on a controller-timed schedule with hard mL/min caps. A whole reservoir dumped is recoverable; a whole 5-gallon bucket dumped usually isn't.
Can I use kalk in a tank with low evaporation?: Less effectively. Kalk's Ca/alk ratio is fixed by stoichiometry — you can't add more without overdosing one or the other. Low-evap tanks (<0.5 % of volume/day) often run two-part with kalk as an occasional supplement.
Hot or cold mixing water?: Cold dissolves slightly more kalk than warm, but the difference is small. Use RO/DI at room temp, mix to saturation (you'll see undissolved powder), let it settle 12 h, and draw from the top — never the sediment.

Where to buy

BRS pharma-grade calcium hydroxide is the standard kalk source. One pound makes ~50 gal of saturated kalk and lasts most tanks 6–12 months.

Kalkwasser (calcium hydroxide) — Mrs. Wages or Two Little Fishies

Drip in top-off for cheap Ca + alk + pH boost.

View on Amazon →

Affiliate links. As an Amazon Associate, reefcalcs earns from qualifying purchases.

Share / embed this calculator

Paste this snippet into any blog or forum post that allows raw HTML to embed the calculator.

<iframe
  src="https://reefcalcs.com/embed/kalk-drip/"
  width="100%"
  height="720"
  style="border: 1px solid #1C2D34; border-radius: 8px; max-width: 720px;"
  loading="lazy"
  title="ReefCalcs Kalkwasser Drip Rate Calculator"
></iframe>

Sources & references

01
Randy Holmes-Farley — Two-part calcium and alkalinity additive system (saturation data)
https://reefs.com/magazine/aquarium-chemistry-a-homemade-two-part-calcium-and-alkalinity-additive-system/
02
Bulk Reef Supply — Kalkwasser: everything you need to know
https://www.bulkreefsupply.com/content/post/kalkwasser-everything-you-need-to-know
03
Reefkeeping Magazine — Limewater (Holmes-Farley)
http://reefkeeping.com/issues/2003-08/rhf/feature/index.php

Last reviewed 2026-05-25