Brief Look Into Calcium
If your corals are going to grow, they need calcium. That part’s simple enough. Stony corals, both SPS and LPS, pull calcium and carbonate out of the water and combine them into calcium carbonate, which is what their skeletons are made of. Clams and coralline algae do the same thing. No calcium, no growth.
Most reefers target somewhere between 400 and 450 ppm. Natural seawater sits around 420, so that’s a good reference point. The difference between the ocean and your tank is volume. The ocean has functionally infinite water. Your tank has maybe a hundred gallons, and every coral in it is pulling calcium out constantly. You have to put it back.
The thing that tripped me up early on is that calcium doesn’t exist in a vacuum. You can’t just watch one number. Calcium, alkalinity, and magnesium all interact with each other, and reefers call them “the big three” for a reason. Magnesium keeps calcium and carbonate from reacting too fast. If your mag is low, you’ll chase calcium and alk numbers all week and never figure out why they won’t hold. I spent a frustrating couple of weeks doing exactly that before I thought to test magnesium. Keeping all three stable matters way more than nailing any one specific number.
What happens when calcium is off
When calcium drops too low, coral growth stalls. You might also notice skeletal tips looking thinner or more brittle. On the other end, too much calcium can cause carbonate to precipitate out of the water, which tanks your alkalinity and can stress corals enough to cause tissue loss. Neither extreme is good, but in my experience the low side is what you’ll run into more often since corals are constantly consuming it.
How to keep it up
I use two-part dosing because it’s simple and predictable: one bottle for calcium, one for alkalinity, dosed in equal amounts. It’s not exciting, but it works and I can put it on a dosing pump and mostly forget about it.
I looked at calcium reactors before going the two-part route. They dissolve a calcium-rich media using CO2, which replenishes calcium and alkalinity together. Once dialed in they’re pretty hands-off, but the upfront cost and tuning turned me away for my setup.
Kalkwasser was another one I tried briefly. It bumps calcium, alk, and pH all at once, which sounds great until you dose too much and your pH spikes hard. It also doesn’t touch magnesium or trace elements, so you’re still supplementing those on the side.
If you have a smaller tank or a lighter coral load, manual dosing works fine too. Test, calculate, add. The downside is it’s easy to forget or get sloppy with it, which brings me to the real point.
Consistency over perfection
I test at least weekly, more like every few days since my tank is mostly SPS. You want to catch changes before they snowball. But the actual numbers matter less than you’d think. A reading that’s a little off but steady is way better than one you’re constantly yanking around with big corrections. I’d rather sit consistently at 410 than bounce between 380 and 460 trying to hit 440. The corals don’t care about perfection. They care about stability.
Sources
- Reef Builders - “The Importance of Calcium, Alkalinity, and Magnesium in a Reef Tank” by Michael Paletta (2023): reefbuilders.com
- Charterhouse Aquatics - “Understanding Calcium (Ca) in a Reef Tank” (2024): charterhouse-aquatics.com
- Fauna Marin - “Calcium” knowledge base article: faunamarin.de
- Manta Systems - “The Essential Role of Calcium in Coral Health”: mantasystems.net
- The Biota Group - “Reef Dosing: Understanding the Balance of Calcium, Alkalinity, pH, and Magnesium” (2024): thebiotagroup.com
- Hanna Instruments - “How to Test Calcium in Your Aquarium” (2022): hannainst.com