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.

Calculator

Kalkwasser Calculator

Kalkwasser is calcium hydroxide — Ca(OH)₂ — dissolved into RO/DI water at saturation, then dripped or pumped in to replace evaporation. At ~1.5 g per litre of solubility, saturated kalkwasser holds roughly 800 ppm Ca and 2 meq/L alkalinity simultaneously. Drip it slowly and a 1 % daily evaporation rate adds about 1.1 dKH and 8 ppm Ca per day. The only chemistry on this site that nudges pH up as a side effect, which is occasionally what you want and occasionally a disaster — see warnings below.

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

1.10dKH/day

Ca supplied

8.0ppm/day

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

How this is calculated

solubility   = 1.5 g Ca(OH)₂ per L water (25 °C)
saturated_Ca = ~800 ppm Ca
saturated_Alk= ~2.0 meq/L = ~5.6 dKH

daily_dose_gal = tank_gal × pct_evap_per_day / 100
alk_supplied   = 1.1 dKH/day per 1 % evap
ca_supplied    = 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.

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Sources & references

01
Bulk Reef Supply — Using Kalkwasser (saturation and dosing)
https://www.bulkreefsupply.com/content/post/using-kalkwasser
02
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/
03
Seachem — Reef Kalkwasser product label (cross-reference)
https://www.seachem.com/reef-kalkwasser.php

Last reviewed 2026-05-25