Comparison
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 (Randy): neutral. Soda ash: rises. 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 is a $20 upgrade.
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.
Where to buy
Kalkwasser (lime water)
Kalkwasser (calcium hydroxide) — Mrs. Wages or Two Little Fishies
Drip in top-off for cheap Ca + alk + pH boost.
View on Amazon →
AutoAqua Smart ATO Micro
Best small-tank ATO. Dual-optical sensor failsafe.
View on Amazon →
Two-part / Calcium reactor
BRS Pharma Sodium Bicarbonate (alkalinity component)
Pre-baked NaHCO₃ for Randy/BRS two-part. Skip the oven step.
View on Amazon →
BRS Pharma Calcium Chloride (Ca component)
Anhydrous CaCl₂ for two-part dosing. Use Randy's recipe math.
View on Amazon →
ESV B-Ionic Two-Part
Pre-mixed liquid two-part if you don't want to DIY.
View on Amazon →
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Related calculators & references
Sources & references
- 01Randy Holmes-Farley — Reef calcification methodshttp://reefkeeping.com/issues/2005-04/rhf/index.php
Last reviewed