Two-Part Dosing, Explained — From First Drop to Dialed In

The Reef Tank Dosing Calculator handles the math. This article is the why.

Why "two-part"

Coral skeletons are calcium carbonate. To grow, corals consume Ca²⁺ and HCO₃⁻ (the alkalinity ion) in a 1:1 molar ratio from the water column. Replenishing them in solution as a single mixture would cause immediate precipitation in the bottle. So we split into two parts:

• Part 1 (Alkalinity): sodium bicarbonate or sodium carbonate solution.
• Part 2 (Calcium): calcium chloride solution.

Dosed separately to the tank, they dissociate into Ca²⁺ and HCO₃⁻ without precipitating. The sodium and chloride ions enter the salt balance — Cl⁻ already dominates, so the load is invisible up to a few mL/day per gallon.

Brands differ in concentration, not in chemistry

Bulk Reef Supply's pharma-grade is the most concentrated common option. Red Sea Reef Foundation is more dilute by design — bigger volumes per dKH raised, but more forgiveness on dose timing. Tropic Marin Pro Reef sits between. Aquaforest adds trace elements.

The choice mostly comes down to dose volume tolerance. Reefkeeper with a 50-gallon tank can manage 5 mL/day of BRS easily; same reefkeeper with a 200-gallon SPS-heavy tank dosing 60 mL/day of Red Sea will want the more concentrated BRS to keep dose volumes manageable.

The magnesium dependency

Magnesium is a precipitation inhibitor in the calcium-carbonate system. Below ~1250 ppm Mg, your dosed Ca and alkalinity will precipitate out of solution as scale instead of being absorbed by corals. You see white residue on heaters and powerheads. Parameter readings stay flat despite consistent dosing.

The fix: raise Mg gradually (50 ppm/day max) using a dedicated magnesium supplement. Tropic Marin Magnesium and BRS Magnesium are both straightforward magnesium chloride/sulfate blends. Don't try to raise Mg by overdosing the standard two-part — Mg is in lower concentration in two-part than Ca, so you'd have to dose enormous volumes.

Alkalinity burn — the 1.4 dKH/day rule

Coral tissue is sensitive to rapid alkalinity changes. An SPS tank that runs at 8 dKH and gets dosed up to 9.5 dKH within hours can develop tissue necrosis at the bases of branches — Slow Tissue Necrosis (STN) starts; if conditions don't recover, Rapid Tissue Necrosis (RTN) sweeps the colony in 24-48 hours.

The industry rule is no more than 1.4 dKH change per day. For a correction that needs more than 1.4 dKH of raise, split it over consecutive days. The calculator flags this automatically.

Why dosing pumps beat manual dosing

Two-part can be added manually — pour the dose into a high-flow area twice a day — but stability suffers. Each dose causes a temporary spike at the dose location before circulation evens it out. Corals near the spike experience the parameter overshoot; corals far away experience the average.

A peristaltic dosing pump (Bubble Magus, Jebao, Kamoer) delivers small doses every few hours. 6 mL × 4 doses/day causes much less of a localized swing than 24 mL × 1 dose/day. For SPS-heavy tanks, this is the difference between thriving and surviving.

Pumps cost $80-150 for a single-channel; the value is so disproportionate to the cost that almost every serious reefkeeper has one within their first year.

When to graduate to a calcium reactor

If your daily uptake exceeds 2.5 dKH, two-part starts to feel like a chore. You're filling 5-gallon containers of additive every two weeks. Sodium and chloride from the additive start to noticeably push salinity (though water changes correct it).

A calcium reactor dissolves aragonite media using CO₂-acidified water, producing a saturated calcium-carbonate solution that drips into the tank. Up-front cost is higher ($300-800 for the reactor, $200 for the regulator, $150 for the pH controller) but operating cost is lower than continuous two-part purchases at high uptake rates.

Rule of thumb: under 1.5 dKH/day uptake, two-part is fine. Over 2.5 dKH/day, calcium reactor pays back within a year. Between 1.5 and 2.5, it's preference.

Test, dose, test, repeat

Two-part dosing without consistent testing is gambling. Test alkalinity and calcium daily for the first two weeks of any new dosing regime; once stable, weekly. A Hanna alkalinity colorimeter gives the most repeatable readings; Salifert is accurate but more user-dependent.

Use the calculator, then adjust to your tank

The calculator gives you a starting dose based on your brand and consumption profile. Run it for a week. Test daily. Adjust by ±10% if your alkalinity is drifting up or down. Within a month, your tank will be parameter-stable enough that you barely think about it.