Kalkwasser (lime water) vs Two-part / Calcium reactor
The three main calcium and alkalinity supply methods compared. Kalkwasser drips with top-off, two-part doses via pump, calcium reactor dissolves aragonite media. Each has a sweet spot.
The contenders
Side A
Kalkwasser (lime water)
Ca(OH)₂ in top-off
Side B
Two-part / Calcium reactor
See below for breakdown
Spec-by-spec
| Spec | Kalkwasser (lime water) | Two-part / Calcium reactor |
|---|---|---|
Daily uptake capacity | Limited by evaporation rate (~1–2 dKH/day) | Two-part: ~10 dKH/day per pump. Reactor: unlimited |
Equipment cost | Kalk reactor or stirrer: $80–$300 | Two-part: $80–250. Reactor: $400–1,500 |
Monthly cost (100 gal SPS) | ~$1–3 (Ca(OH)₂ is cheap) | Two-part: $15–35. Reactor: $3–8 |
pH effect | Raises pH (caustic, dosed slowly) | Two-part: soda ash / baked baking soda raises it, unbaked bicarbonate barely moves it. Reactor: lowers (CO₂) |
Magnesium handling | No — separate | Two-part: separate. Reactor: partial |
Complexity | Low (gravity feed, slow drip) | Two-part: low. Reactor: high |
Risk | pH spike if dosed too fast | Two-part: pump failure. Reactor: pH controller failure → CO₂ runaway |
Which one for which situation
Small reef <50 gal, just starting dosing
Pick: Kalkwasser (lime water)
Kalk top-off is the cheapest, simplest entry. You're already topping off — adding lime water costs a ~$30 tub of powder.
Mid-size (50–150 gal) mixed reef
Pick: Two-part / Calcium reactor
Two-part covers this range cleanly. Reactor is overkill until uptake exceeds kalk + 2-part capacity.
SPS-dominant >200 gal
Pick: Two-part / Calcium reactor
Calcium reactor. Two-part bottle changes become a chore at this volume.
Tank pH drops every night below 7.9
Pick: Kalkwasser (lime water)
Kalk dosed on a reverse photoperiod boosts night pH. The combination of kalk + ATO is the original pH-stabilization trick.
Existing 2-part user, want lower monthly cost
Pick: Kalkwasser (lime water)
Add kalk to your top-off. Each 1 ppm Ca delivered via kalk is one you don't dose via 2-part.
Which method should you start with?
Affiliate linksreefcalcs pick under 50 gal
Two Little Fishies Kalkwasser (calcium hydroxide) — 2 kg
You're already topping off, so lime water is a ~$30 tub of powder that covers Ca and alk at once and props up night-time pH. Cheapest entry into dosing there is.
~$30 · Where to buy →Required to dose kalk
Tunze Osmolator 3155 ATO
Kalk only works delivered slowly through top-off. Hand-pouring it is how people spike pH and burn corals — and a stuck single-sensor ATO dumping saturated kalk is worse, so use one with an optical sensor and a backup float.
~$190 · Where to buy →Once you outgrow kalk
BRS Pharma Soda Ash (alkalinity component)
Kalk saturates out around 1–2 dKH/day of demand. Past that, two-part is the next step up and covers the 50–150 gal mixed-reef range cleanly.
~$25 · Where to buy →Affiliate links. As an Amazon Associate, reefcalcs earns from qualifying purchases.
Also mentioned above
Affiliate links. As an Amazon Associate, reefcalcs earns from qualifying purchases.
Related calculators & references
Sources & references
- 1.Randy Holmes-Farley — Reef calcification methodshttp://reefkeeping.com/issues/2005-04/rhf/index.php
Last reviewed