Comparison
Calcium reactor vs Two-part dosing
Calcium reactors vs two-part dosing compared on uptake range, monthly cost, complexity, and what happens when things go wrong. Pick by consumption rate, not by hobby trend.
The contenders
Side A
Calcium reactor
Media-based, CO₂ + aragonite
Side B
Two-part dosing
Liquid Ca + alk pumps
Spec-by-spec
| Spec | Calcium reactor | Two-part dosing |
|---|---|---|
Daily uptake supported | Unlimited (effluent-rate driven) | Practical limit ~10 dKH/day per dosing pump |
Startup cost | $400–$1,500 (reactor + CO₂ + regulator + pH controller) | $80–$250 (3 pumps + bottles) |
Monthly consumable cost (100 gal SPS) | ~$3–8 (CO₂ + media refills) | ~$15–35 (Ca + alk + Mg) |
Effluent variability | Drifts with pH; needs monitoring | Fixed — pump volume is what gets dosed |
Add magnesium? | Partial — media has some, may need supplement | Yes — third bottle (e.g. Recipe 3A) |
Failure modes | pH probe drift, CO₂ runaway → pH crash | Pump miscalibration, empty bottle → no dose |
Learning curve | High — pH controller, media break-in, effluent tuning | Low — calc, fill, pump |
Which one for which situation
Mixed reef <100 gal, mostly LPS
Pick: Two-part dosing
Uptake is low enough that two-part stays cheap and you skip the reactor learning curve.
SPS-dominant >150 gal, daily uptake >3 dKH
Pick: Calcium reactor
Reactor consumables cost a fraction of equivalent two-part. ROI on the reactor inside 12 months.
First reef tank, less than 1 year experience
Pick: Two-part dosing
Two-part teaches you how alkalinity and calcium move in your tank. Reactor obscures the underlying chemistry.
Frag tank for growing out SPS commercially
Pick: Calcium reactor
Reactors scale better and don't run out of supplement mid-day.
Vacation-proof setup, owner gone 2+ weeks
Pick: Calcium reactor
Properly-tuned reactors run for weeks on a CO₂ tank and bag of media. Two-part needs liquid refills.
Where to buy
Calcium reactor
Salifert KH/Alkalinity test kit
The hobbyist standard for alkalinity testing.
View on Amazon →
Salifert Calcium test kit
Pairs with the alk kit for two-part dosing tracking.
View on Amazon →
Two-part dosing
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 →
Affiliate links. As an Amazon Associate, reefcalcs earns from qualifying purchases.
Related calculators & references
Sources & references
- 01Randy Holmes-Farley — Reef chemistry methodshttp://reefkeeping.com/issues/2005-04/rhf/index.php
Last reviewed