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Reef Dosing Calculator
Work out how much alkalinity, calcium or magnesium supplement to dose, scaled from your product's own stated strength to your tank's volume and the gap between your current and target readings. Runs entirely in your browser — no login, no account, nothing saved or sent anywhere.
Alkalinity, calcium and magnesium dosing calculator
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Why alkalinity, calcium and magnesium matter
These three are grouped together because they're chemically linked, not because they're interchangeable. Corals, clams, coralline algae and other calcifying invertebrates build their skeletons from dissolved calcium and carbonate (the ion alkalinity measures), so both are drawn down from the water as those animals and algae grow[2]. Seawater carries enough dissolved calcium and carbonate to support this, but only because magnesium slows how readily calcium carbonate crystallises out of solution — without enough magnesium present, calcium and alkalinity become harder to keep dissolved at the same time, and the two can precipitate out as scale rather than staying available for corals to use[3].
Research on coral calcification specifically has found that calcification rate tends to increase with the seawater's calcium-carbonate (aragonite) saturation state[2] — broadly, more available calcium and carbonate, kept in solution, means more material for calcifying animals to build with. This is research on saturation state generally, from an experimental coral-reef mesocosm, not a study of home reef-aquarium target numbers specifically, but it's consistent with why reef keepers treat these three parameters as a linked system rather than three unrelated readings.
None of this applies to a fish-only marine tank with no calcifying livestock — see the FAQ below. For testing methods, target-range detail and the early warning signs of an imbalance, see our full reef chemistry guide.
Starting points, not targets
Commonly cited target ranges
These reflect widely repeated conventions among reef keepers, broadly consistent with natural seawater — not a single scientifically mandated figure for every tank. Stability within a sensible range matters at least as much as hitting an exact number.
| Parameter | Commonly cited range | Notes |
|---|---|---|
| Alkalinity | 8–12 dKH (roughly 2.9–4.3 meq/L, or 143–214 ppm as CaCO3) | Check which unit your own test kit reports — dKH, ppm and meq/L are all genuinely different numbers for the same reading. |
| Calcium | 380–450 ppm | Consumed alongside alkalinity as corals build their skeletons. |
| Magnesium | 1,250–1,400 ppm | Levels much below roughly 1,000 ppm make it harder to keep calcium and alkalinity stable together; levels much above roughly 1,600 ppm are generally considered unnecessary and are approached cautiously. |
Only reef tanks with corals or other calcifying invertebrates generally need to actively manage these three. A fish-only marine tank doesn't need this level of attention to any of them — see our starting a saltwater aquarium guide for the difference between the two setups.
Dosing methods, briefly
There's no single best method for every tank — each involves a genuine trade-off between simplicity, cost and consistency.
Common dosing mistakes
- Correcting a large gap in one dose. A sudden, large swing in any of these three is generally more stressful to corals than a gradual correction — see the FAQ below.
- Dosing without retesting. A product's stated strength is itself an approximation, and test kits carry their own margin of error — retest before dosing again rather than stacking doses on assumption.
- Dosing calcium and alkalinity separately without watching magnesium. If magnesium runs low, calcium and alkalinity become harder to keep stable together — see "Why these matter" above.
- Dosing a fish-only tank with no calcifying livestock. Without corals or similar invertebrates consuming these parameters, there's generally nothing to correct for — regular water changes with a quality salt mix are usually enough.
- Assuming a different product's stated strength applies to yours. Dosing strength genuinely varies between brands and formulations — always use your own product's label, not a figure from somewhere else.
How AquaPulse helps
Once you've worked out a dose, AquaPulse can help with the ongoing side of keeping a reef tank stable — not the initial calculation:
- Recording readings over time, so you can see alkalinity, calcium and magnesium alongside each other rather than checking each in isolation.
- Trend monitoring, so a slow drift is easier to notice than it would be from occasional, isolated tests.
- Testing and dosing reminders, so a regular routine doesn't depend entirely on memory.
- Ecosystem context that takes your aquarium type into account, rather than treating every tank the same way.
AquaPulse provides informational guidance only, based on what you enter — it doesn't monitor your water automatically or replace testing with a reliable kit. Get AquaPulse on the App Store or Google Play to try it.
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Related tools and guides
Common questions
Frequently asked questions
Do I need to dose alkalinity, calcium and magnesium in every saltwater tank?
No. These three parameters matter because corals and other calcifying invertebrates (such as certain clams, snails and coralline algae) build their skeletons from them. A fish-only marine tank with no calcifying livestock generally doesn't need active dosing at all, and regular water changes with a quality marine salt mix are usually enough to replace what's lost. This calculator is aimed at reef tanks that keep corals or similar invertebrates.
What's a safe target range for alkalinity, calcium and magnesium in a reef tank?
There's no single number that's officially correct for every system. Widely cited hobby conventions suggest roughly 8-12 dKH for alkalinity, 380-450 ppm for calcium, and 1,250-1,400 ppm for magnesium, broadly consistent with natural seawater. Stability within a sensible range matters at least as much as hitting an exact figure, and it's generally more useful to choose a target and hold it steady than to chase small changes.
Why do calcium and alkalinity drop together?
Corals and other calcifying invertebrates build calcium carbonate skeletons by combining calcium and carbonate ions from the water in a fixed ratio, so both are consumed together as they grow. A widely cited approximate ratio is around 20 ppm of calcium consumed for every 1 meq/L of alkalinity consumed (roughly 7 ppm calcium per 1 dKH) — useful for understanding why the two move together, though actual consumption in your tank also depends on your livestock and growth rate, not this ratio alone.
How much should I change these parameters at once?
Gradually. For alkalinity specifically, many reef keepers limit changes to roughly 0.5-1 dKH per day, since rapid swings — even in the right direction — are more stressful to corals than a slower correction. Similar caution generally applies to calcium and magnesium: if this calculator suggests a large single dose, consider splitting it across several days and retesting between doses rather than correcting the whole gap at once.
What if I don't know my product's dosing strength?
Check the product's own label or manufacturer's instructions — most alkalinity, calcium and magnesium supplements state something like "this much product raises the reading by this much, in this much water." This calculator scales that figure to your own tank and target change; it doesn't assume any particular brand's dosing strength, since these vary between products.
Which dosing method should I use — two-part, kalkwasser, or a calcium reactor?
There's no single best choice for every tank. Two-part liquid dosing is generally the simplest to start with and suits most reef tanks; kalkwasser (limewater) can supply alkalinity and calcium together but needs careful, slow addition because of its high pH, and generally suits low-to-moderate demand; a calcium reactor gives very consistent dosing with less manual top-up but costs more and takes longer to set up correctly. Higher, more consistent demand — a heavily stocked reef with fast-growing corals — generally favours more automated methods, but this is a general pattern, not a rule for every tank.
How this calculator works: the dose is calculated directly from your product's own stated strength, scaled by simple ratio to your system volume and the gap between your current and target readings — it does not model tank chemistry or predict consumption from first principles. Volume unit conversions use exact published constants (1 US gallon = 3.785411784 litres, 1 UK gallon = 4.54609 litres). The alkalinity unit conversions (1 dKH = 17.86 mg/L as CaCO3 = 1/2.8 meq/L) are standard unit definitions, used only to flag a large requested change, not to convert your dose calculation itself. Nothing you enter is saved or sent anywhere.
What we deliberately left out: a single universal "safe" target number (there isn't one — see the table above); a calculation of how much your specific corals will actually consume (this depends on livestock, growth rate and lighting far too much to model reliably); and how to lower a reading that's already too high, which is a different problem (usually a water change, reduced dosing, or addressing an overdose) that this calculator doesn't attempt to solve.
Sources and editorial information
- Published
- 27 July 2026
- Last reviewed
- 27 July 2026
- Written and maintained by
- AquaPulse (Valeon Labs Ltd)
This calculator and its supporting content are written and maintained by the AquaPulse team at Valeon Labs Ltd. The dosing arithmetic uses your own product's stated strength rather than an assumed universal figure; the target ranges reflect widely repeated hobby convention rather than a single official standard, and are presented as such throughout. We paraphrase and link to sources rather than reproducing their wording, and we'll update this page if we find something we've got wrong.
Good to know: This page provides general informational guidance only. It is not a substitute for testing your own water with a reliable kit, or for specialist aquatic-health advice for corals or other invertebrates showing signs of distress.
- [1] Merck Veterinary Manual — "Environmental Diseases of Aquatic Animals in Aquatic Systems" Used for the general role of alkalinity in buffering pH and supporting biological processes in aquatic systems, consistent with this source's use elsewhere on AquaPulse (see our saltwater aquarium guide). Covers aquatic systems broadly, not reef-aquarium-specific dosing.
- [2] Langdon, Takahashi, Sweeney et al. (2000), "Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef," Global Biogeochemical Cycles 14(2), 639–654 Accessed via a USGS-hosted copy. Used for the general relationship between seawater calcium-carbonate (aragonite) saturation state and coral calcification rate. This is an experimental mesocosm study of saturation state broadly, not a study of home reef-aquarium target parameters, and no specific numeric target from it is quoted in this page.
- [3] "The role of Mg2+ in inhibiting CaCO3 precipitation from seawater," Marine Chemistry (2021) Publisher listing page only (abstract), not the full text. Used only for the general, long-established chemical mechanism by which magnesium slows calcium-carbonate precipitation in seawater — a widely corroborated finding, not a specific result quoted from this paper's own data. General marine chemistry research, not aquarium-specific, and not the source of any target range figure on this page.