How does the calcium dose compare to the alkalinity dose?

Randy's Recipe #1 is built so equal volumes of the two parts match what coral consumes. 1 mL per US gal raises Ca ~9.8 ppm and alk ~1.4 dKH — about 7 ppm Ca per dKH, close to the 7.15 ppm the CaCO₃ stoichiometry predicts. Moving a 50-gal by 20 ppm Ca takes ~102 mL of Ca stock; the same tank needs ~36 mL of alk stock per 1 dKH. A Ca correction looks like more liquid only because 20 ppm Ca is the equivalent of nearly 3 dKH.

Should I dose calcium separately or use two-part?

Two-part. Calcium chloride alone leaves you with sodium and chloride imbalance over time. Pairing calcium dosing with sodium bicarbonate/carbonate (the alk side) means the ions balance: 2 NaHCO₃ + CaCl₂ → CaCO₃ (consumed by coral) + 2 NaCl (the salt you started with). Water changes purge whatever drift accumulates.

What if my Ca is high but alk is low?

Don't drop calcium — raise alk and let coral consumption pull Ca down naturally. Calcium drops slowly. Removing it actively (by water changes or letting it precipitate) is wasteful and uncontrolled.

Calcium Dosing Calculator — Randy CaCl₂, BRS, Red Sea, Triton

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.

Calcium dosing

Tank volume (US gal)

Product / recipe

Current Ca (ppm)

Target Ca (ppm)

Required change

30.0ppm Ca

Liquid dose

153.46mL

500 g calcium chloride dihydrate dissolved to 1 US gal. Stock ≈ 37,000 ppm Ca. Use about 25 % less by mass (≈377 g) if using anhydrous CaCl₂.

Source: reefs.com

How much CaCl₂ that actually isAffiliate link

Calcium Chloride, food grade — 7 lb

Burning ~225 g/month, one 7 lb pack covers ~14 months · ~$30.

Based on a 75 gal (284 L) mixed reef consuming 1.0 dKH/day, which pulls about 7.15 ppm of calcium with it, dosed as calcium chloride dihydrate (CaCl₂·2H₂O). Run your own numbers above — demand scales with tank volume and coral load.

Food-grade calcium chloride in a bucket — the cheapest way to run the calcium side of a DIY two-part, as long as you check the hydration state before doing the recipe math.

View on Amazon →

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

Gear for thisAffiliate links

BRS Pharma Calcium Chloride (Ca component)~$25 · View on Amazon →

Hanna Marine Calcium Checker (HI758)~$50 · View on Amazon →

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

How this is calculated

Randy Recipe #1 Ca stock = 500 g CaCl₂·2H₂O → 1 gal solution
                       ≈ 37,000 ppm Ca = 37 mg Ca per mL

mL = (tank_L × ΔCa_ppm) / 37
   = tank_gal × ΔCa_ppm × 3.785 / 37
   ≈ tank_gal × ΔCa_ppm × 0.1023

BRS Pharma's published reference (100 gal × 20 ppm = ~102 mL stock) lines up with the math: about 0.051 mL/gal/ppm. Red Sea Foundation A is weaker: 1 mL per 100 L raises Ca by 2 ppm. Triton's dry correction is 3 g anhydrous CaCl₂ per 100 L per 10 ppm.

Anhydrous calcium chloride needs about 25 % less by mass than the dihydrate (CaCl₂·2H₂O) because water of hydration is dead weight (molar mass 111 vs 147 g/mol). If your source ships anhydrous, drop the stock recipe from 500 g to about 377 g per gal — concentration is the same.

FAQ

How does the calcium dose compare to the alkalinity dose?: Randy's Recipe #1 is built so equal volumes of the two parts match what coral consumes. 1 mL per US gal raises Ca ~9.8 ppm and alk ~1.4 dKH — about 7 ppm Ca per dKH, close to the 7.15 ppm the CaCO₃ stoichiometry predicts. Moving a 50-gal by 20 ppm Ca takes ~102 mL of Ca stock; the same tank needs ~36 mL of alk stock per 1 dKH. A Ca correction looks like more liquid only because 20 ppm Ca is the equivalent of nearly 3 dKH.
Should I dose calcium separately or use two-part?: Two-part. Calcium chloride alone leaves you with sodium and chloride imbalance over time. Pairing calcium dosing with sodium bicarbonate/carbonate (the alk side) means the ions balance: 2 NaHCO₃ + CaCl₂ → CaCO₃ (consumed by coral) + 2 NaCl (the salt you started with). Water changes purge whatever drift accumulates.
What if my Ca is high but alk is low?: Don't drop calcium — raise alk and let coral consumption pull Ca down naturally. Calcium drops slowly. Removing it actively (by water changes or letting it precipitate) is wasteful and uncontrolled.

Where to buy

Affiliate links

BRS Pharma calcium chloride for DIY two-part. ESV B-Ionic if you want pre-mixed. Salifert is the standard Ca test kit.

BRS Pharma Calcium Chloride (Ca component)CaCl₂·2H₂O (dihydrate) flake for two-part dosing. Matches Randy's recipe math.View on Amazon →ESV B-Ionic Two-PartPre-mixed liquid two-part if you don't want to DIY.View on Amazon →Salifert Calcium test kitPairs with the alk kit for two-part dosing tracking.View on Amazon →Red Sea Reef Foundation Pro test kitCa, Mg, alk in one box. Designed for Red Sea's reef recipes.View on Amazon →

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

Share / embed this calculator

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<iframe
  src="https://reefcalcs.com/embed/calcium-dosing/"
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  style="border: 2px solid #111412; max-width: 720px;"
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Sources & references

1.
Randy Holmes-Farley — Two-part recipe (calcium side)
https://reefs.com/magazine/aquarium-chemistry-a-homemade-two-part-calcium-and-alkalinity-additive-system/
2.
Bulk Reef Supply — BRS Pharma Calcium Chloride mixing
https://www.bulkreefsupply.com/content/post/brs-pharma-calcium-chloride-mixing-and-dosing-instructions
3.
Red Sea — Foundation Complete Manual
https://redseafish.com/wp-content/uploads/2020/11/24653_Manual-Foundation-Complete-GB-_v21a-WEB.pdf
4.
Triton — CoreChem (CaCl₂ correction)
https://www.triton.de/en/products/corechem

Last reviewed 2026-09-30