Reef Tank Dosing Calculator

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.

Worked example: 90-gallon mixed reef on Red Sea

Take a 90-gallon mixed reef on Red Sea Reef Foundation — strengths from our data file: alk 0.55 dKH, Ca 5.5 ppm, Mg 9.0 ppm, each per mL per gallon. Tests read 7.2 dKH / 400 Ca / 1280 Mg; targets are 8.5 / 440 / 1350; medium consumption is 1.0 dKH and 10 ppm Ca per day:

• Alk correction: (8.5 − 7.2) × 90 ÷ 0.55 = 212.7 mL of Foundation B
• Ca correction: (440 − 400) × 90 ÷ 5.5 = 654.5 mL of Foundation A
• Mg correction: (1350 − 1280) × 90 ÷ 9.0 = 700 mL of Foundation C
• Daily maintenance: 1.0 × 90 ÷ 0.55 ≈ 163.6 mL alk plus 10 × 90 ÷ 5.5 ≈ 163.6 mL Ca per day

Note that the 1.3 dKH rise slips just under the 1.4 dKH/day burn threshold — start at 7.1 instead of 7.2 and the calculator makes you split the correction over two days. And 1280 ppm Mg clears the 1250 ppm precipitation floor only barely: spread the Foundation C over two or three days and re-test.

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.

Brand strengths compared

Strengths below are normalized from each manufacturer's published dosing chart to one convention — parameter rise per 1 mL per US gallon — with the practical upshot in the fourth column.

Common mistakes

• Correcting alkalinity in one shot. A 3 dKH correction is three days of dosing, not one big pour — past ~1.4 dKH/day you are trading a low number for burned SPS tissue.
• Dosing both parts at the same point. Carbonate meeting calcium at full strength precipitates instantly. Opposite ends of the sump, high flow, 10-15 minutes apart.
• Chasing alk and Ca while magnesium is low. Below ~1250 ppm the doses become scale on your heater and the test numbers never move. Magnesium first, always.
• Trusting an aging test kit. Reagents drift as they age, and a 0.5 dKH kit error rewrites your whole schedule. Verify against a certified reference solution every few months.

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.

Sources & how we keep this current

Every number in this tool traces to a named source:

• Manufacturer dosing charts — Bulk Reef Supply's two-part instructions and calculator, Red Sea's Reef Care Program charts (Foundation A/B/C), Tropic Marin's Pro Reef A+B dosage tables, and Aquaforest's Component guide supply the per-mL strengths, normalized to per-US-gallon in our data file.
• Randy Holmes-Farley's reef chemistry series (Advanced Aquarist, Reef2Reef) — the magnesium precipitation floor and safe alkalinity ramp rates behind the 1250 ppm and 1.4 dKH/day thresholds.
• Published salt-mix parameter sheets — the Coral Pro, Tropic Marin Pro Reef, Aquaforest, and natural-seawater targets cited in the FAQ.

Strengths were last verified 2026-05-21; the date is stored beside the numbers in the data file. Manufacturers reformulate — if a new bottle's label disagrees with this table, trust the label, and confirm every output against your own test trend.