Two-part dosing, explained

Why two-part

Coral builds skeleton from calcium carbonate. Every gram of skeleton it deposits removes one Ca²⁺ ion and one CO₃²⁻ (carbonate) ion from the water. In a stocked reef tank with growing coral, you measurably watch both calcium and alkalinity drop on the same days, in the same molar ratio. If you replace them, the coral keeps growing. If you don't, calcium falls to 360 ppm, alkalinity to 6 dKH, growth stops, and the corals start losing tissue.

Other methods exist. Kalkwasser — saturated calcium hydroxide solution — supplies both in the same molar ratio that they're consumed, but capped by a daily evaporation ceiling. Calcium reactors dissolve aragonite media with CO₂ injection — elegant and zero-maintenance once tuned, but expensive and twitchy. All-in-one balanced bottles like Tropic Marin AFR and Red Sea Foundation work, just at 4–6× the cost per dKH delivered.

Two-part is what most ambitious hobbyists end up running. Bulk powders cost roughly a dollar per dKH-month for a 100-gallon tank. The chemistry is transparent, the dose is auditable, and a dosing pump failure is reversible. The alkalinity dosing calculator and the matching calcium dosing calculator apply the recipe directly.

The stoichiometry

When coral builds CaCO₃ skeleton, the reaction is:

Ca²⁺ + 2 HCO₃⁻ → CaCO₃ + CO₂ + H₂O

One mole of calcium removed per two moles of bicarbonate. In hobby units: 20 ppm Ca consumed equals approximately 2.8 dKH consumed. If you're losing 1 dKH per day, you're also losing ~7.15 ppm Ca per day. That linked ratio is why we dose two parts in equal volumes — equal-volume dosing keeps the ionic balance stable as coral grows.

The trick is that you can't dose Ca²⁺ and CO₃²⁻ from the same bottle. They'd precipitate as CaCO₃ in the jug. So you keep them separate. Part 1 is the calcium side; Part 2 is the alkalinity side. They meet in the diluted water column where the concentration product is far below the solubility threshold for spontaneous precipitation.

The recipe

Randy's Recipe 1 uses the cheapest, most defensible ingredients:

Part 1: Calcium

Calcium chloride dihydrate (CaCl₂·2H₂O). Dissolve 500 grams in 1 gallon of RO/DI water. Yields a stock that delivers approximately 37,000 ppm Ca. Pharmaceutical grade is sold by BRS, Bulk Supplements, or Esco; food-grade pickling-quality calcium chloride works identically. Reagent grade is overkill.

Part 2: Alkalinity

Sodium bicarbonate (NaHCO₃) — baking soda. Two options here:

• Raw baking soda: dissolve 670 grams per gallon. Delivers ~1900 meq/L stock as bicarbonate. Slightly depresses pH on dosing because bicarbonate is a weaker base than carbonate.
• Baked baking soda — soda ash: bake regular baking soda at 300 °F for 1 hour to drive off water and CO₂, converting NaHCO₃ → Na₂CO₃. Then dissolve 594 grams baked product per gallon. Delivers the same ~1900 meq/L but raises pH because carbonate is the stronger base. This is the Randy Recipe 1 default.

Bulk Reef Supply sells the same chemicals pre-portioned as BRS Pharma Soda Ash and BRS Pharma Calcium Chloride. The math is identical because the chemistry is identical.

Dosing math

For Recipe 1 stocks: 1 mL of Part 2 per gallon of tank raises alkalinity by 0.135 dKH. 1 mL of Part 1 per gallon raises calcium by 9.8 ppm. The 1:1 dosing ratio of Part 1 to Part 2 keeps the molar consumption balanced. The arithmetic:

mL Part 2 per day = (dKH consumed per day) × (tank gallons) / 0.135 mL Part 1 per day = mL Part 2 per day  (equal volumes)

A 100-gallon tank losing 1 dKH per day needs (1 × 100) / 0.135 ≈ 741 mL of Part 2 per day. That's a lot. In practice, hobbyists make stocks at half-strength when starting out so a dosing pump misfire is half as catastrophic. Half-strength stocks just double the daily volume.

Split the daily dose. Randy recommends at least four doses per day — every six hours — so individual additions don't spike the local concentration high enough to precipitate. Dosing pumps make this trivial. A 24×1-mL/hour schedule is the modern best practice on tanks with consumption above 2 dKH per day.

Magnesium part 3

Two-part doesn't add magnesium. Magnesium gets consumed slowly by coral skeleton formation (Mg substitutes into the aragonite lattice at roughly 5% by weight) and by routine precipitation in the tank. Without a separate magnesium dose, Mg drifts down over weeks; once Mg falls below ~1,200 ppm, calcium and alkalinity start precipitating at the rates you're adding them and your dosing "stops working."

Randy's Mg recipe uses a 4:1 weight blend of magnesium chloride hexahydrate to magnesium sulfate heptahydrate. The chloride/sulfate ratio matches natural seawater so you don't accumulate ionic imbalance. Dissolve 720 g MgCl₂·6H₂O + 180 g MgSO₄·7H₂O in 1 gallon RO/DI. BRS sells the same blend pre-mixed.

Yield: 1 mL of this stock per gallon raises Mg by approximately 26.6 ppm. The magnesium dosing calculator computes exact volumes. Test Mg weekly and bump only when you see it drift below 1,300 ppm.

Salinity creep

Two-part adds sodium (from Part 2) and chloride (from Part 1) in addition to the calcium and bicarbonate you want. Over months, sustained dosing measurably raises tank salinity. Randy estimates ~1.5 ppt salinity rise per year of heavy two-part dosing on a typical SPS tank. That's significant — you'd drift from 35 ppt to 37 ppt in 18 months.

Water changes fix this. A 10% monthly water change with 35 ppt salt is sufficient to keep salinity drift below 0.5 ppt per year on most tanks. If you don't do water changes (some folks run Triton or zero-water-change systems), salinity creep is real and you have to watch for it.

Common mistakes

Mistake 1: Skipping the bake on soda ash

Plain baking soda works but it depresses pH because bicarbonate is the weaker base. On a tank already running pH 7.8 from a basement CO₂ load, switching to baked product gains you 0.1–0.2 pH units of ceiling. The bake takes an hour at 300 °F in a sheet pan; do it once for a year's supply.

Mistake 2: Equal-volume dosing without measuring

The 1:1 Part 1 to Part 2 ratio depends on your tank's actual consumption. If your tank consumes Ca and Alk in a different molar ratio (which can happen with heavy coralline algae growth, which deposits high-Mg calcite at different stoichiometry), 1:1 dosing will pull one parameter up and the other down. Test Ca and Alk weekly. Adjust the ratio if you see persistent drift in one direction.

Mistake 3: Dosing into low flow

Dose lines should drip into high-flow areas — at the return pump, near a powerhead, into a sump baffle with current. Dripping concentrated Part 2 onto stagnant rockwork precipitates a white film of CaCO₃ before it disperses. The film is harmless but ugly, and it's a sign you're wasting alkalinity.

Mistake 4: Treating "part 2 raises pH" as a feature

Soda-ash dosing does briefly raise pH where the drip lands. On tanks with chronically low pH (basement CO₂), this is occasionally marketed as a benefit. It isn't — the localized pH spike can precipitate CaCO₃ at the drip point. If you have a pH problem, fix it at the source (CO₂ scrubber, fresh-air intake to the skimmer). Don't use two-part as a pH workaround.

When to graduate to a calcium reactor

Below 1 dKH/day consumption, two-part is fine forever. Above 2 dKH/day, you're mixing 5-gallon jugs constantly and salinity creep starts being a maintenance item. That's the practical inflection where a calcium reactor justifies the $400–$700 capital cost. Heavy SPS tanks over 200 gallons usually end up there.

Until then: weigh the chemicals, dose the volumes, test the numbers. The math doesn't change with the price of the system you mix it in.