Alk / Ca / Mg Dosing Calculator
Enter your current and target levels, pick your supplement, and get the exact dose. No more guessing — raise alkalinity, calcium, or magnesium safely and precisely.
Calculate Your Dose
Actual water volume, not tank size (typically 70-80% of display volume)
Select a parameter and supplement to see dosing information.
Target Ranges at a Glance
These are widely accepted target ranges for reef aquariums. Stability within your chosen target matters more than hitting a specific number.
| Parameter | NSW | Acceptable Range | Ideal Target | Max Daily Change |
|---|---|---|---|---|
| Alkalinity | 7 dKH | 7 – 11 dKH | 8 – 9 dKH | 1 dKH |
| Calcium | 420 ppm | 380 – 450 ppm | 420 ppm | 20 ppm |
| Magnesium | 1350 ppm | 1250 – 1400 ppm | 1350 ppm | 50 ppm |
NSW = Natural Seawater. For more detail, see our Big 3 guide.
Dosing Tips
Go Slow
Never raise alkalinity more than 1 dKH/day, calcium more than 20 ppm/day, or magnesium more than 50 ppm/day. Split large corrections across multiple days.
Dissolve First
Always dissolve dry supplements in RO/DI water before adding to your tank. Never add powder directly — it creates localized pH spikes that can burn corals.
Fix Mg First
If magnesium is low, raise it before adjusting calcium or alkalinity. Low magnesium destabilizes the other two — you'll chase numbers endlessly until Mg is in range.
Test After Dosing
Wait 1–2 hours after dosing before re-testing. This allows the supplement to fully mix and react in your tank water for an accurate reading.
Spread It Out
For daily dosing, split the total into multiple smaller doses throughout the day. A dosing pump running 6–12 times daily is far better than one manual dump.
Log Everything
Track your test results and dosing amounts over time. Trends reveal consumption rate changes before they become problems. ReefTanker makes this easy.
Track Your Parameters Over Time
The calculator gets you to your target. ReefTanker keeps you there — log results, spot trends, and dial in stability.