How do I mix kalk safely?

Wear gloves and eye protection — Ca(OH)₂ is alkaline enough to irritate skin and eyes. Add powder to cold RO/DI, stir vigorously, let it settle, then draw clear solution off the top. Don't aerate while stirring — exposing fresh kalk to air converts it to CaCO₃, which is insoluble and useless.

Convenience for hobbyists without a scale. 1 tsp loose Ca(OH)₂ is roughly 4 g (varies 3.5–5 g by brand and packing). Use weight when you can.

Can I overdose kalkwasser?

Yes — both directions. Too fast a drip from saturated solution can spike pH past 8.6 and cause Ca / PO₄ precipitation. Above-saturation (more powder than 1.5 g/L) just leaves undissolved powder at the bottom and adds nothing extra to the water.

Does kalk work for big tanks?

Up to a point. A 200-gal tank evaporating 2 gal/day needs 7.5 L of saturated kalk daily — workable. A 500-gal evaporating 5 gal/day needs almost 19 L; that's a 5-gallon reservoir refilled every day. Most large tanks switch to a calcium reactor or two-part.

Kalkwasser Calculator — Saturated Limewater Daily Dose

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

Tank volume (US gal)

Daily evaporation (% of system)

Typical reef: 0.5–2 %/day.

Daily kalk volume

0.50US gal

Daily kalk volume

1.89L

Alk supplied

1.12dKH/day

Ca supplied

8.0ppm/day

Solubility: 1.5 g Ca(OH)₂ per litre. Saturated kalkwasser at ~800 ppm Ca and ~40 meq/L (≈ 112 dKH) alkalinity.

How much kalk powder that actually isAffiliate link

Two Little Fishies Kalkwasser (calcium hydroxide) — 2 kg

Burning ~180 g/month, one 2 kg pack covers ~11 months · ~$30.

Based on a 75 gal (284 L) reef evaporating about 1 gal/day, dripped at saturation (~6 g of Ca(OH)₂ per gal of top-off). Run your own numbers above — demand scales with tank volume and coral load.

Saturated lime water in your top-off covers calcium and alkalinity at once and props up night-time pH — the cheapest dosing method there is.

View on Amazon →

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Gear for thisAffiliate links

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

Tunze Osmolator 3155 ATO

~$190 · View on Amazon →

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How this is calculated

solubility   = 1.5 g Ca(OH)₂ per L water (25 °C)
saturated_Ca = ~800 ppm Ca
saturated_Alk= ~40 meq/L = ~112 dKH  (2 meq per mmol Ca(OH)₂)

daily_dose_gal = tank_gal × pct_evap_per_day / 100
alk_supplied   = 112 dKH / 100 ≈ 1.1 dKH/day per 1 % evap
ca_supplied    = 800 ppm / 100 = 8 ppm/day per 1 % evap

Above 2 % evaporation per day, replacing all of it with saturated kalkwasser can drive pH past 8.55 — coral don't love that, fish handle it but not happily. Cap kalk at half your daily makeup and supplement the rest with regular two-part if your evaporation is high. Run kalk overnight only if your tank pH dips at night (most do) — it offsets CO₂ accumulation while you sleep.

Phosphate precipitation is the bonus feature. High pH pulls free phosphate down into Ca-phosphate complexes that drop out into the sand bed. Tanks using kalkwasser as primary Ca/Alk often run unmeasurably low PO₄ without GFO. That's a feature for SPS dominant tanks, a problem for soft-coral / LPS heavy systems that want some phosphate.

FAQ

How do I mix kalk safely?: Wear gloves and eye protection — Ca(OH)₂ is alkaline enough to irritate skin and eyes. Add powder to cold RO/DI, stir vigorously, let it settle, then draw clear solution off the top. Don't aerate while stirring — exposing fresh kalk to air converts it to CaCO₃, which is insoluble and useless.
Why teaspoons?: Convenience for hobbyists without a scale. 1 tsp loose Ca(OH)₂ is roughly 4 g (varies 3.5–5 g by brand and packing). Use weight when you can.
Can I overdose kalkwasser?: Yes — both directions. Too fast a drip from saturated solution can spike pH past 8.6 and cause Ca / PO₄ precipitation. Above-saturation (more powder than 1.5 g/L) just leaves undissolved powder at the bottom and adds nothing extra to the water.
Does kalk work for big tanks?: Up to a point. A 200-gal tank evaporating 2 gal/day needs 7.5 L of saturated kalk daily — workable. A 500-gal evaporating 5 gal/day needs almost 19 L; that's a 5-gallon reservoir refilled every day. Most large tanks switch to a calcium reactor or two-part.

Where to buy

Affiliate links

Kalk powder dosed via ATO is the cheapest dosing method per dKH — you're already topping off, so the ATO does the work. Use a two-sensor unit: AutoAqua Smart ATO Duo for nano, Tunze Osmolator for mid-size, Brute can for the reservoir.

Two Little Fishies Kalkwasser (calcium hydroxide) — 2 kgDrip in top-off for cheap Ca + alk + pH boost.View on Amazon →AutoAqua Smart ATO DuoTwo optical sensors in one small housing. The nano ATO with a real backup.View on Amazon →Tunze Osmolator 3155 ATOMid-size ATO with optical + float failsafe.View on Amazon →Rubbermaid Commercial Brute 14 gal containerStandard ATO reservoir. Food-grade plastic.View on Amazon →Salifert KH/Alkalinity test kitThe hobbyist standard for alkalinity testing.View on Amazon →

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

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Sources & references

1.
Bulk Reef Supply — Using Kalkwasser (saturation and dosing)
https://www.bulkreefsupply.com/content/post/using-kalkwasser
2.
Randy Holmes-Farley — Kalkwasser pros and cons (Reef2Reef thread)
https://www.reef2reef.com/threads/optimal-parameters-for-a-coral-reef-aquarium-by-randy-holmes-farley.173563/
3.
Seachem — Reef Kalkwasser product label (cross-reference)
https://www.seachem.com/reef-kalkwasser.php

Last reviewed 2026-09-30