If you've spent any time in reef keeping forums or communities, you've heard the phrase "keep your parameters stable." Two parameters sit at the very top of that list: calcium and alkalinity. Together they form the chemical foundation that your corals use to build their skeletons — and getting them right is the difference between thriving, fast-growing corals and ones that slowly decline.
Why Calcium and Alkalinity Matter
Stony corals (both SPS and LPS) build their skeletons from calcium carbonate (CaCO₃). To do this, they consume two things from your water simultaneously: calcium ions (Ca²⁺) and carbonate alkalinity (dKH). Think of calcium and alkalinity as two sides of the same coin — you can't supplement one without managing the other.
Natural seawater parameters:
- Calcium: 380–420 ppm (target: 400–420 ppm)
- Alkalinity: 8–9.5 dKH (target: 8–10 dKH)
- Magnesium: 1250–1350 ppm (supports both; test monthly)
The Calcium-Alkalinity Relationship
Calcium and alkalinity have an inverse relationship — raising one can push the other down. More importantly, they precipitate out of solution when both are elevated beyond their natural balance. This is why you should never dose calcium and alkalinity supplements directly into the same area of your sump at the same time.
Dosing Methods
1. Two-Part Dosing (Best for Beginners)
Two-part systems use two separate liquid solutions — Part A (alkalinity) and Part B (calcium) — dosed in equal amounts daily. They're easy to dial in, don't require equipment beyond a dosing pump, and work well for tanks up to ~150 gallons with moderate coral load.
- Start by testing your baseline parameters
- Calculate your tank's daily consumption
- Dose equally and test every 2–3 days until stable
- Automate with a dosing pump for consistency
2. Kalkwasser (Limewater)
Kalkwasser (German for "lime water") is a calcium hydroxide solution dripped slowly into your tank, typically through your top-off water. It raises both calcium and alkalinity simultaneously while also raising pH. It's cost-effective but requires careful dripping — too fast and you'll spike your pH dangerously high.
3. Calcium Reactor
A calcium reactor uses CO₂ to dissolve calcium carbonate media, releasing calcium and alkalinity into your tank water naturally. It's the gold standard for large, heavily stocked SPS tanks — highly efficient, low cost per unit once set up, but requires CO₂ equipment and regular pH monitoring.
How to Start Dosing
- Test first. Know your baseline before adding anything. Test calcium, alkalinity, and magnesium.
- Correct magnesium first if it's below 1250 ppm — low magnesium prevents calcium and alkalinity from staying stable.
- Raise alkalinity slowly — no more than 0.5–1 dKH per day to avoid shocking corals.
- Match your calcium target once alkalinity is stable.
- Monitor weekly at minimum. SPS-heavy tanks may need testing every 2–3 days.
Ready to Build Your Reef?
Stable parameters unlock the full potential of your coral collection. Browse our coral collection and our SPS corals — perfect for reefers who have their chemistry dialed in and are ready to level up.