Reef Tank Dosing Calculator — Alkalinity, Calcium, Magnesium

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Red Sea Reef Foundation A/B/C
Three-bottle balanced ionic system. More forgiving on dose timing; better for new reefers.
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Hanna Alkalinity Checker (HI772)
Accurate testing is non-negotiable. The Hanna colorimeter beats visual color-match kits for repeatability.
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Bubble Magus dosing pump
Quietest reliable peristaltic dosers; 1, 3, or 4-channel models.
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Salifert test kits
The budget standard for calcium and magnesium checks between alkalinity tests.
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How the math works

Each brand has a strength factor: the parameter increase produced by 1 mL of additive in a 1-gallon tank. The dose for a real tank is then mL = change × gallons ÷ strength. With BRS two-part (strength ≈ 1.0 dKH per mL per gallon), raising a 100-gallon tank by 1 dKH takes about 100 mL; replacing a consumption of 2 dKH/day takes about 200 mL/day. A less concentrated commercial brand like Red Sea Reef Foundation needs roughly double the volume for the same job.

The calculator splits the dose into a one-time correction (bring current to target) and an ongoing daily maintenance (replenish consumption). These are layered: dose the correction over 3-5 days to avoid alkalinity burn, then settle into the daily maintenance.

Alkalinity burn — the 1.4 dKH/day rule

Coral tissue is sensitive to rapid alkalinity changes. A jump from 7.5 to 9.5 dKH within hours causes tissue necrosis (STN/RTN) in SPS species. The industry rule of thumb is no more than 1.4 dKH/day of alkalinity change. The calculator flags when your correction dose would exceed this.

The Mg dependency

Magnesium acts as an inhibitor of calcium carbonate precipitation. Below ~1250 ppm Mg, dosed Ca and alk precipitate as scale instead of being absorbed by living tissue. You see this as white residue on equipment and as flat parameter readings despite consistent dosing. Always correct Mg first.

Brand selection

BRS pharma-grade is the most concentrated common option (smallest dose volumes). Red Sea Fdn is more dilute (larger volumes, more forgiveness on timing). Tropic Marin sits between. Aquaforest is geared toward complete-system reefkeepers (trace elements integrated). Kalkwasser is its own category — alkalinity and calcium delivered simultaneously via top-off; capacity-limited by evaporation.

When this calculator is the wrong tool

Use a calcium reactor for daily uptake exceeding 2.5 dKH (the calculator will flag this). Use a dosing controller (Apex, GHL) for tanks where alkalinity stability matters within 0.1 dKH. This tool sizes the doses; precision delivery requires equipment beyond manual measurement.

Related guide

Two-Part Dosing, Explained From first drop to dialed in — brand differences, Mg dependency, alk burn, dosing pumps.

FAQ

Questions, answered

How do I know my consumption rate?

Test alkalinity at the same time each day for a week, recording the dose you administered between tests. The difference between (yesterday's dose × strength) and (today's measurement - yesterday's measurement) is your consumption. For SPS-heavy tanks, 1.0-2.5 dKH/day is typical.

What if my Mg is low?

Correct Mg first. Below ~1250 ppm, dosed calcium and alkalinity precipitate as calcium carbonate scale instead of being absorbed by corals. You'll see white residue on heaters and powerheads. Raise Mg gradually (target +50 ppm/day) before resuming alk/Ca dosing.

Kalkwasser vs two-part?

Kalkwasser delivers alkalinity and calcium simultaneously in a perfect 1:1 ionic ratio — better for ionic balance than two-part. But it can only be added via top-off (replaces evaporated water), so capacity is limited by your evaporation rate. Two-part is more flexible; kalkwasser is more elegant when it works.

Why do brands differ so much in dose volume?

Concentration. BRS pharma-grade is a saturated solution; Red Sea Fdn is more dilute by design (to reduce shock from large doses). Picking a brand is a trade-off between dose volume (smaller for concentrated) and forgiveness (larger for dilute).

How often should I dose?

For SPS-dominant tanks, 4-6 events per day via dosing pump — stability matters as much as the total. For LPS / softies, once or twice manually is fine. Never dump a large single dose: even within the daily-burn threshold, a single big dose causes a localized parameter spike.