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.

A worked example: the 75-gallon mixed reef

Say you run a 75-gallon display. Rock and sand displacement put actual water volume closer to 65 gallons (about 246 liters) — dosing math should always use net volume, not the sticker on the tank. Pause dosing for 24 hours and test at the same time each day: alkalinity falls from 8.6 to 8.2 dKH. Daily consumption is 0.4 dKH.

One dKH equals 0.357 meq/L of alkalinity (17.9 ppm calcium-carbonate equivalent), so the tank consumes 0.4 × 0.357 × 246 ≈ 35 meq per day. Dry sodium carbonate (soda ash) supplies about 18.9 meq per gram, so 1.9 g/day replaces it. Plain sodium bicarbonate supplies 11.9 meq per gram, so you would need 3.0 g/day instead.

Because corals pull calcium and carbonate in a 1:1 molar ratio, balanced consumption pairs every 1 dKH of alkalinity with roughly 7.2 ppm of calcium — the stoichiometric ratio Randy Holmes-Farley's Reefkeeping Magazine chemistry articles built the DIY two-part system around. Your 0.4 dKH/day tank is therefore also consuming about 2.9 ppm of calcium daily, roughly 2.6 g of calcium chloride dihydrate. If the two sides drift apart over weeks of logging, something other than coral growth is eating one of them — usually abiotic precipitation from low magnesium.

The dosing calculator runs these conversions per brand, so you get an answer in milliliters rather than milliequivalents.

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.

Common mistakes — and what experienced reefkeepers do instead

• Dosing both parts at the same time and place. Concentrated Ca²⁺ meeting concentrated carbonate turns the dose to snow instead of keeping it in solution. Offset pump schedules by at least 30 minutes and dose at opposite ends of the sump.
• Chasing a single test result. Hobby titration kits resolve to roughly ±0.3 dKH at best. Experienced keepers adjust on a three-day trend, in ±10% steps, never on one reading.
• Never calibrating the dosing pump. Peristaltic tubing hardens with age and output drifts low over months. Calibrate monthly into a graduated cylinder; replace tubing yearly.
• Dosing alkalinity at midday. Schedule alkalinity doses overnight instead: soda ash gives a temporary pH lift that offsets the dark-cycle pH sag from respiration CO₂ — one schedule change smooths both curves.

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.

Common questions

Can I mix both parts in one container?

No. Combined in concentrated solution, calcium chloride and sodium carbonate precipitate immediately as insoluble calcium carbonate — that is the entire reason the system is two-part. The same applies to hardware: never repurpose an alkalinity dosing line for calcium without a vinegar rinse first, or residue plates the tubing shut.

Baking soda or soda ash for the alkalinity part?

Both work, and Holmes-Farley's classic DIY recipe accommodates either. Sodium bicarbonate has minimal immediate pH effect; sodium carbonate temporarily raises pH. The DIY conversion is baking bicarbonate on a sheet pan at about 300 °F for an hour, which drives off water and CO₂. If your tank runs chronically low pH — 7.8 or below overnight — the soda-ash lift is a feature. If it already runs high, bicarbonate is the safer pick.

Does two-part replace water changes?

No. Two-part replenishes calcium and alkalinity — plus magnesium in three-part systems — but not trace elements, and it slowly adds sodium and chloride that corals never consume. Routine water changes reset both. Skipping them because the big three parameters look perfect is the most common way a dosed tank quietly drifts out of ionic balance.

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.