Reef Water Parameters Explained: Salinity, Alkalinity, Calcium and Magnesium
By Melbourne Tropical Team Β· 2 min read
Stable water is everything in a reef tank. Here's what salinity, alkalinity, calcium and magnesium do - and the target numbers to aim for.
Why stability beats perfection
Corals can adapt to a slightly 'off' number if it is steady, but a swinging parameter will stress or kill them. The goal of reef chemistry is consistency: pick sensible targets and keep them stable with regular testing and dosing.
The key parameters and targets
These are the numbers most reef keepers aim for:
- Salinity: 1.024-1.026 specific gravity (about 35 ppt) - measure with a refractometer.
- Temperature: 24-27 Β°C, kept steady.
- pH: 8.1-8.4.
- Alkalinity (KH): 8-12 dKH - the buffer that keeps pH stable and feeds coral skeletons.
- Calcium: 400-450 ppm - the main building block of coral skeletons.
- Magnesium: 1300-1400 ppm - keeps calcium and alkalinity in balance.
- Ammonia & nitrite: 0. Nitrate: low (under 10 ppm, under 5 for SPS). Phosphate: very low.
How they work together
Alkalinity, calcium and magnesium are linked. Corals consume calcium and alkalinity as they grow, so in a busy reef you top them up by dosing. Magnesium is the 'helper' that keeps calcium and alkalinity stable - if magnesium is low, you'll struggle to hold the other two. Test these three together and dose to keep them steady. Use our parameter converter for unit conversions.
Testing and dosing routine
Test alkalinity most often (it moves fastest), plus calcium and magnesium weekly in a coral-heavy tank. Soft-coral and fish-only tanks need far less fuss; SPS-dominated tanks need tight, frequent control. Always make changes slowly - never try to correct a parameter in one big jump.