Corals build their skeletons from calcium and carbonate taken from the water. In a growing reef tank these elements are used up every day, and keeping them stable is what separates thriving corals from struggling ones.
Target ranges
- Alkalinity (KH): 7–9 dKH for most mixed reefs; SPS tanks often run 7.5–8.5. Stability matters more than the exact number.
- Calcium: 400–450 ppm
- Magnesium: 1250–1400 ppm
- Salinity: 35 ppt (SG ≈ 1.026)
Why stability matters
Swings hurt corals more than slightly-off values. An alkalinity change of more than about 0.5 dKH in a day can cause tissue loss in SPS corals. Aim for steady daily dosing rather than large corrections.
Test first, in the right order
- Magnesium first. Low magnesium makes it very hard to keep calcium and alkalinity stable.
- Then calcium and alkalinity. Test at the same time of day, and not right after your dosing pump runs.
- Measure daily consumption: test alkalinity at the same time on two consecutive days without dosing.
How to raise them safely
- Alkalinity: sodium bicarbonate (baking soda) raises it gently; sodium carbonate works faster and raises pH more. Raise by at most 1 dKH per day.
- Calcium: calcium chloride dihydrate. Raise by at most 20 ppm per day.
- Magnesium: magnesium chloride hexahydrate, ideally mixed about 5:1 with magnesium sulfate. Raise by at most 100 ppm per day.
As a rule of thumb, in 100 L of water about 3 g of sodium bicarbonate raises alkalinity by 1 dKH, and about 3.7 g of calcium chloride dihydrate raises calcium by 10 ppm. Fishtify’s reef calculator works it out for your net water volume.
Keeping up with demand
- Water changes replace some elements in small, young tanks.
- Two-part dosing (alkalinity + calcium) with a dosing pump suits most growing reefs.
- Kalkwasser added with top-off water supplies both and helps raise pH.
- Calcium reactors suit heavily stocked SPS tanks.
When values are too high
Stop dosing that element and let the corals use it up. High alkalinity combined with very low nitrate and phosphate is a known cause of burnt SPS tips, so keep nutrients in range too; see how to manage nitrate.
Fishtify tracks alkalinity, calcium and magnesium against targets for the corals you keep and warns you when the daily swing is too large.