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.

Kalkwasser Drip Rate Calculator — mL/min, ppm Ca, dKH per day

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

10.7ppm

Alk added per day

1.49dKH

From saturated kalk at 25 °C (~40 meq/L ≈ 112 dKH).

Saturated kalk: ~800 ppm Ca and ~40 meq/L (≈ 112 dKH) alkalinity at 25 °C (Holmes-Farley). Replacing 1 % of tank volume adds ~8 ppm Ca and ~1.1 dKH. Drip via a peristaltic or aquadoser into a high-flow area, never directly onto coral.

Gear for thisAffiliate links

Two Little Fishies Kalkwasser (calcium hydroxide) — 2 kg ~$30 · View on Amazon →

Tunze Osmolator 3155 ATO

~$190 · View on Amazon →

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

How this is calculated

Saturated kalkwasser (25 °C, well-stirred):
  Ca  ≈ 800 ppm  (≈ 20 mmol/L Ca(OH)₂)
  Alk ≈ 40 meq/L (2 meq per mmol)  ≈ 112 dKH

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
  Alk_dKH  = (mL_per_day ÷ 1000 × 112 dKH) ÷ tank_L

  1 % of tank volume per day → +8 ppm Ca, +1.1 dKH

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

The 800 ppm Ca figure is from Randy Holmes-Farley's calcium-hydroxide solubility data; the alkalinity follows from it, because every mmol of dissolved Ca(OH)₂ carries two meq of hydroxide alkalinity — 40 meq/L, or about 112 dKH. Those are the same numbers the kalkwasser calculator uses, so the two agree. 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

Affiliate links

Two Little Fishies calcium hydroxide is a widely stocked kalk source. Saturated limewater takes about 6 g of powder per gallon of top-off, so the 2 kg tub makes roughly 300 gal of kalk.

Two Little Fishies Kalkwasser (calcium hydroxide) — 2 kgDrip 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: 2px solid #111412; max-width: 720px;"
  loading="lazy"
  title="reefcalcs Kalkwasser Drip Rate Calculator"
></iframe>

Sources & references

1.
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/
2.
Bulk Reef Supply — Kalkwasser: everything you need to know
https://www.bulkreefsupply.com/content/post/kalkwasser-everything-you-need-to-know
3.
Reefkeeping Magazine — Limewater (Holmes-Farley)
http://reefkeeping.com/issues/2003-08/rhf/feature/index.php

Last reviewed 2026-09-30