Free tool
Reef dosing calculator
Your alkalinity, calcium or magnesium is low — how much do you actually add? Tell us your water volume, your test result and your target, and we'll work out the dose from the real chemistry, then split it safely over as many days as it needs. Test, don't guess.
⚠️ Dose safely — this is the part that kills tanks.
- Always dissolve the chemical fully in RO/DI water first, then add it slowly to a high-flow spot — never dump dry powder in the tank.
- Test first, and test again before re-dosing. A dose is only as good as the test behind it.
- Don't dose calcium and alkalinity at the same time or the same spot — mixed at high concentration they precipitate back out (and can cloud the tank).
- Get magnesium into range (~1300 ppm) first — calcium and alkalinity won't hold if magnesium is low.
- Weigh it. Teaspoon "guesstimates" vary a lot by product; a cheap kitchen scale is the "test, don't guess" of dosing.
An honest tool, not a promise. Every dose here is derived from the chemical's exact stoichiometry — the same public chemistry the BRS Reef Calculatorand Randy Holmes-Farley's published recipes use — and each product line states the exact compound and hydrate it assumes. Your product may differ (purity, hydrate, or a pre-mixed liquid), so always cross-check against its own label. This raises a parameter; to lower one, do a water change.
Sources & method
- Constants derived from standard molar chemistry, benchmarked to the canonical figure (~1 tsp baking soda raises ~50 US gal by ~1 dKH).
- Randy Holmes-Farley — Optimal Parameters for a Coral Reef Aquarium (targets + reef chemistry method).
- Bulk Reef Supply — Pharma dosing instructions (safe daily limits: calcium ≤50 ppm/day; magnesium ≤25 g/80 L/day; dissolve first, dose slowly, test again).
A free Coralkin tool — no sign-up, nothing saved. It does the arithmetic; your test kit and your product's label have the final say. We never guess a care number.