Dosing a Reef Tank: Alk, Calcium, Magnesium
🐠 The short version: as stony corals grow, they use up alkalinity, calcium and magnesium faster than water changes can replace — so you dose. Measure your tank’s daily consumption, dose exactly enough to keep each number flat, and respect the safe daily limits. Two hard rules: never dose calcium and alkalinity together (they precipitate), and bring magnesium up first. Steady beats perfect.
🔧 The dosing calculator turns your water volume and target rise into a precise, stoichiometry-derived dose — no guesswork.
When you actually need to dose
Not every tank needs dosing. A fish-only, soft-coral, or lightly-stocked tank is usually kept topped up by regular water changes alone. Dosing becomes necessary when growing stony corals (LPS and especially SPS) consume alkalinity, calcium and magnesium faster than water changes return them. The tell is a measurable daily drop in alkalinity (and calcium) between changes.
Measure your demand first
Don’t dose blind — find your tank’s appetite:
- Test a parameter (start with alkalinity).
- Wait ~24 hours without dosing it.
- Test again. The difference is your daily consumption.
Now you dose exactly that much per day to hold the number flat. This is the whole game — you’re replacing what corals use, not chasing a target up and down.
The three, and how they relate
All from the sourced parameter chart:
- Alkalinity — 8–11 dKH (SPS tanks often 7–8). The carbonate buffer corals build skeleton from, and the one they’re most sensitive to.
- Calcium — 400–450 ppm. The other main skeleton block.
- Magnesium — 1300–1400 ppm. The balancer: it keeps calcium and alkalinity in solution together. If magnesium is low, you’ll fight to hold the other two — so bring magnesium up first.
Dosing safely
The rules that keep dosing from becoming a crash:
- Respect daily limits (from our dosing calculator, sourced to Randy Holmes-Farley / Bulk Reef Supply): raise alkalinity ≤ ~1 dKH/day (conservatively ~0.5; BRS lists ~1.5 as a max), calcium ≤ ~50 ppm/day, magnesium ≤ ~35 ppm/day. Spread bigger corrections over several days.
- Never dose calcium and alkalinity together — combined, they precipitate, clouding the water and wasting both. Dose them at different times or different points, into good flow.
- Dissolve/weigh accurately — measure by weight, not eyeballed scoops; the calculator’s numbers assume real quantities.
- Dose into flow, not onto corals.
Two-part vs kalkwasser vs calcium reactor
Three delivery methods for the same job:
- Two-part — alkalinity and calcium (plus magnesium) from separate bottles. Simple, precise, ideal for most tanks. Start here.
- Kalkwasser (limewater) — added via top-off, replaces alk and calcium together; helps stabilise pH.
- Calcium reactor — for high-demand SPS systems; more kit, more consistency at scale.
Match the method to demand: two-part for most, reactor only when a hungry SPS tank outgrows it.
Stability over perfection
Dosing exists to hold numbers steady, which is what corals actually want — a steady value beats a perfect one that swings, and alkalinity swings are a leading cause of tank crashes and RTN/STN. Size your doses to your measured demand, keep magnesium in range, dose alk and calcium apart, and let consistency do the work.
Sources & further reading
- Coralkin — Dosing calculator — stoichiometry-derived doses and the cited daily caps.
- Coralkin — Reef parameter chart and water chemistry explained.
- Coralkin — Preventing a reef tank crash — why steady alkalinity matters most.
- Randy Holmes-Farley reef chemistry + Bulk Reef Supply — the dosing and daily-limit basis.
Common questions
When do I need to start dosing?
When your growing stony corals consume alkalinity, calcium and magnesium faster than regular water changes replace them. A lightly-stocked or soft-coral tank usually does not need dosing at all. The signal is measurable daily drop in alkalinity (and calcium) between water changes as your SPS and LPS grow.
How do I know how much to dose?
Measure your tank's daily consumption first: test a parameter, wait 24 hours without dosing, and test again - the difference is your daily uptake. Dose enough to replace exactly that, so the number stays flat. The dosing calculator turns your volume and target rise into a precise amount.
What are the safe daily dosing limits?
As a guide, raise alkalinity by no more than about 1 dKH per day (many keepers stay conservative at around 0.5), calcium by up to about 50 ppm per day, and magnesium by up to about 35 ppm per day. Bigger corrections should be spread over several days - fast changes stress corals.
Can I dose calcium and alkalinity at the same time?
No. Dosed together or into the same spot at the same moment, calcium and alkalinity precipitate out (they combine and cloud the water, wasting both). Dose them at different times or different points in the tank, into good flow, and bring magnesium up first so the other two stay in balance.
Whats the difference between two-part, kalkwasser and a calcium reactor?
They are three ways to replace the same things. Two-part dosing adds alkalinity and calcium (plus magnesium) from separate bottles - simple and precise for most tanks. Kalkwasser (limewater) tops off and adds alk/calcium together. A calcium reactor suits high-demand SPS systems. Start with two-part; scale up only if demand outgrows it.