To raise calcium in a reef tank, dose calcium chloride dissolved in RO/DI water. That’s either the DIY kind or a commercial calcium part. It takes about 13.9 grams of calcium chloride dihydrate to raise 100 gallons by 10 ppm. Before you dose, check magnesium and alkalinity, because low magnesium or high alkalinity is often the real reason calcium won’t stay up.
What should calcium be in a reef tank? #
Natural seawater holds about 420 ppm calcium. Randy Holmes-Farley’s reef parameters table recommends 380-450 ppm, and most reef keepers aim for something around 400-440. Stability matters more than hitting a specific number within that range. See our reef tank parameters chart for how calcium fits with the other targets.
Below about 380 ppm, corals and coralline algae calcify more slowly. Above about 450 ppm isn’t usually harmful by itself. But pushing calcium high while alkalinity is also high invites precipitation, which drags both numbers down.
Why won’t my calcium go up? #
Check these before reaching for more supplement:
- Low magnesium. Magnesium keeps calcium and carbonate from precipitating together. Below about 1,250 ppm, calcium and alkalinity both fall no matter how much you add. Fix it with how to raise magnesium in a reef tank.
- High alkalinity. Running alk at 12 dKH or more, especially with high pH from kalkwasser, drives calcium carbonate out of solution onto heaters, pumps and sand.
- Test error. Calcium titration kits need careful drop counting. A single reading 30 ppm off is common, so retest before you dose.
- Real consumption. A growing tank full of stony corals and coralline algae can use a lot of calcium. That calls for a daily dose, not a one-off correction.
How much calcium chloride raises calcium by 10 ppm? #
Calcium chloride dihydrate (CaCl2ยท2H2O) is about 27% calcium by weight. To raise one US gallon by 1 ppm you need 3.785 mg of calcium, which is about 13.9 mg of the dihydrate.
Use net water volume, meaning the tank plus sump minus rock and sand.
| Net water volume | Calcium chloride dihydrate for +10 ppm | For +20 ppm |
|---|---|---|
| 20 gal (76 L) | 2.8 g | 5.6 g |
| 40 gal (151 L) | 5.6 g | 11.1 g |
| 75 gal (284 L) | 10.4 g | 20.9 g |
| 100 gal (379 L) | 13.9 g | 27.8 g |
| 150 gal (568 L) | 20.8 g | 41.7 g |
| 200 gal (757 L) | 27.8 g | 55.6 g |
If your calcium chloride is anhydrous (no water of hydration), it’s more concentrated, and you need about 25% less by weight. Randy’s DIY two-part article says to use about 20% less by volume for anhydrous or monohydrate forms. Whatever you buy, the label should list pure calcium chloride with no additives, anti-caking agents or corrosion inhibitors.
How do you raise calcium step by step? #
- Test calcium, alkalinity and magnesium on the same day. You need all three to decide what to dose.
- Fix magnesium first if it’s low. Calcium added to a low-magnesium tank tends to disappear.
- Pick a target. If you’re at 360 ppm, aim for 420, not 460.
- Calculate and weigh. Use the table above or your product’s label.
- Dissolve completely in RO/DI water. Calcium chloride releases heat as it dissolves, so mix it in a container and let it cool.
- Add slowly to a high-flow area, away from where you dose alkalinity. Adding both in the same place at the same time causes precipitation.
- Split big corrections across several days, testing between doses.
- Retest after the water mixes, and compare with what you expected.
Using a commercial calcium part #
| Product | Rate stated by maker | Amount for +10 ppm in 100 gal |
|---|---|---|
| Randy’s Recipe #2 calcium (about 18,500 ppm) | Calculated | About 204 ml |
| Randy’s Recipe #1 calcium (about 37,000 ppm) | Calculated | About 102 ml |
| Red Sea Reef Foundation A (liquid) | 1 ml per 100 L raises Ca 2 ppm | About 19 ml |
| ESV B-Ionic Calcium | 1 ml per gallon raises Ca 16 ppm | About 63 ml |
These strengths vary widely, so dose from the label on your bottle. Don’t borrow a number from a forum post or another product.
How do you keep calcium stable once it’s up? #
A one-time correction doesn’t help much if the tank uses 10 ppm a day. Once calcium is in range, match what the tank consumes with one of these methods:
| Method | How it works | Good for | Not ideal for |
|---|---|---|---|
| Balanced two-part | Separate calcium and alkalinity doses, by hand or doser | Most mixed reefs | Keepers who won’t test regularly |
| Kalkwasser (limewater) | Calcium hydroxide in top-off water | Low-to-moderate demand, low pH tanks | High-demand SPS tanks (limited by evaporation) |
| Calcium reactor | CO2 dissolves aragonite media | Large, high-demand systems | Small tanks, tight budgets |
| Water changes | New salt carries calcium | Soft coral and FOWLR tanks | Anything calcifying quickly |
How do calcium and alkalinity balance? #
Corals build skeletons from calcium carbonate, so they use calcium and alkalinity in a fixed ratio. About 20 ppm of calcium goes with each 1 meq/L (2.8 dKH) of alkalinity, which is roughly 7 ppm of calcium per dKH. If your tank uses 1 dKH a day, expect it to use about 7 ppm of calcium a day as well.
If alkalinity is steady but calcium keeps sliding, you’re under-dosing calcium relative to alk. The reverse is also common with baking soda-only dosing, covered in raising alkalinity with baking soda.
How do you spot a calcium problem early? #
The pattern shows up in the numbers weeks before corals react. A calcium line creeping down 5 ppm a week is easy to miss on paper and obvious on a chart.
Fish Tank Tracker logs calcium, alkalinity and magnesium together in its Full Test, and plots each against an optimal band (400-450 ppm for calcium, 8-11 dKH for alk, 1,300-1,400 ppm for magnesium). Seeing the three lines side by side makes the ratio problem easier to spot: alk flat, calcium falling means your calcium dose is too small. Use the notes field to record each dose so you can tie changes in the chart to what you added.
Frequently asked questions #
Can water changes raise calcium in a reef tank? #
Yes, if your salt mix makes up to a higher calcium level than your tank water. For lightly stocked tanks, weekly changes may be all you need. For tanks full of stony corals, changes alone rarely keep up.
Is 380 ppm calcium too low for a reef tank? #
It’s at the low end of the usual range but not an emergency. Bring it up gradually toward 400-420 ppm, and check that magnesium and alkalinity aren’t the reason it’s low.
Can I use deicer calcium chloride from a hardware store? #
Some reef keepers do, and Randy’s article names specific products he considered acceptable at the time. Formulations change, so read the current label: it should be calcium chloride only, with no added salts, dyes or corrosion inhibitors. Food-grade or USP-grade calcium chloride removes the guesswork.
Does calcium chloride affect pH or alkalinity? #
Calcium chloride has little direct effect on pH or alkalinity. Kalkwasser is different: it raises calcium, alkalinity and pH together. If you’re adding calcium chloride and alkalinity keeps dropping, the tank is consuming alk, so dose both.
Why is my calcium high but alkalinity low? #
Usually the tank is getting more calcium than alkalinity. That happens after dosing calcium alone, or with a salt mix that runs high in calcium. Stop the calcium dose, raise alkalinity gradually, and let consumption bring calcium down.